To save everyone time from trying to untangle what I think from what they think I think, I will cover my beliefs about the structure and analysis of the series in many contentious areas.
Also to let everyone know how I interpret information transmitted, I take everything at face value and as an "ordinary casual fan" would take it with as little ret-conning as absolutely possible and as few preconceptions as absolutely possible. I will grant that some times, the format and target audience of a show, movie, book, etc. will typically slant this, but I endeavour to remain as neutral as possible.
This means that I don't - indeed, can't - assume either Creddie or Seddie. That also means that I will often wind up with contradictory information because I am painted into a corner by the production staff. What I do then is interpret each individual action at as close to face value as I can while trying to move contradictory interpretations towards each other. You will see this philosphy reflected several times below. I also only go by what is presented within the episode, or movie. Something that a writer says at a con or Dan Schneider says in a tweet does not concern me because an "average casual fan" would not care to have ready access to it.
N.B.: As I wrote this, I realised I should post my canonicity rules that come from long, drawn out, pizza-and-pop-fueled arguments in con suites of numerous science fiction and fantasy conventions. I was notorious for taking the most narrow view, so I will present alternate views as well. That post is next.
You might also notice that this would work against me in any debate within a fandom where I was a BNF. That was fine with me because I wanted the discussions to be more inclusive.
Why do I think the series was designed to end Creddie?
The series is first and foremost a Miranda Cosgrove vehicle. It was designed to work synergistically with her record deal with Columbia Records. The convention in this case is for the lead to get the prime romantic options and for the supporting cast to get the leftovers. The pitch goes roughly that we have one female lead with one male co-lead and one female co-lead. Then the executives then ask if there is any romance planned. The obvious response would be that the female lead could pair with the male co-lead. For Seddiers, at best, the set-up was no ship at the beginning, but with the default being Creddie if they felt like it.
You can also take a look at the videos released by Miranda Cosgrove: ever male lead bears at least a passing resemblance to Nathan Kress at the time. This is Columbia and Nickelodeon working synergistically to make fans of her music to follow iCarly and vice-versa.
Why didn't I like the Creddie ending?
I will leave out my usual spiel about not believing that two people are "meant to be" together.
My principal objection is that the ending sends what I consider to be terrible messages to males and females in the viewing audience who are often at a very impressionable age when they discovered the show.
The first problem is that it makes Freddie look like he dated Sam just to mark time waiting for Carly. It implies that using one of your best friends and manipulating him/her is OK while you wait for someone else.
The second problem is that, frankly, I think it makes Freddie look like a loser for hanging on to Carly. This is how I view myself for hanging on to the first woman I fell in love with and kept thinking about getting back together with her when another relationship didn't work. I didn't really mature until I learnt to move on and deal with my feelings properly. Teaching kids about this skill early would have been very helpful.
The fhird problem is that it condones Carly's manipulation of Freddie in early episodes. Every time Carly uses Freddie with, "Please, for me?" I want to throw a brick through my television. Make no mistake - she may be nicer to him than Sam is eary on, but she uses his feelings shamelessly.
The fourth problem is that it condones the idea that Carly can take Freddie's feelings for granted. The message iGoodbye gives with iSpeed Date is that Carly can date any idiot because Freddie will always take her back. And the message iGoodbye sends with iSaved Your Life is that Carly only feels the need to hook up with Freddie when she might be separated from him either permanently (iSYL) or for a long time (iGB).
The biggest problem is the conflation of all of these in those who are not fans of the show even picking up on these. ALL of my nieces and nephews have noticed changes in behaviour of girls and boys to match the worst of all of these behaviours in their friends.
Why wouldn't I have liked a Seddie ending?
Again, my issue with a Seddie ending is that it would have sent some terrible messages to the viewing audience.
The first problem here is that it would have condoned Sam's treatment of Freddie early on. Make no mistake: she abuses him. I find the idea that she is hiding her affection for him before iKiss flatly risible. She is contemptuous of his guilessness and his OCD crush on Carly.
Secondly, as the series played out, you can interpret the Seddie relationship as abusive because it is implied that Sam hits Freddie every now and then. (Granted, it looks like Freddie lets her, but because that is not explicitly mentioned, I don't assume it.) I can't possibly get behind that.
Thirdly, I like the implications that Sam and Freddie fell in love and it didn't work because their established pre-romantic relationship and their backgrounds made a romantic relationship untenable. It's a good message that people should learn: sometimes, being in love isn't enough.
Lastly, it makes Freddie look like a tool. He can only see his two best friends as potential girlfriends. Really? Maybe they are the best choices for him, but what the heck is that based on?
Why do I think the Creddie kiss in iGoodbye was romantic?
Starting with Carly largely ignoring Freddie's explanation, she then grabs his hand and pulls him towards her. She then kisses him and Freddie puts his arms around her and kisses her back. They then break the kiss and realise that there is no turning back. Freddie then celebrates by putting his hands in the air.
Now, as an ordinary, casual fan, I see that as a 100% romantic interaction. There is nothing in the entire episode aside from Freddie asking Sam out (discussed below) that contradicts this interpretation.
Why do I think that both Freddie's and Sam's declarations of love in "iLove You" are romantic and NOT platonic?
The end of "iCan't Take It" had Carly tell Freddie, "Sam loves you." After prompting from Carly, Sam says, "It's, uh, kind of true." This was to get them back together. The ordinary, obvious interpretation I see an average casual fan making is that: (1) Sam said to Carly (NOT that Carly put words into her mouth) (2) in private (3) beforehand (4) that she loved Freddie (5) "the good way" as Creddiers would put it.
Thus, Freddie should never say, "I love you," unless he means it "the good way" as well. After they date, they make out for 90 minutes until they break up.
Why do I think that Freddie asking Sam out in iGoodbye is genuine?
Because there is no reason for Freddie to bring up that possibility at all unless he was thinking about doing so. If he was worried about Sam bringing it up and how to say "no", then he should not bring it up. Aside from the Creddie kiss later (see above), there is nothing else in the episode that would contradict this interpretation.
If any of the theories that he brought it up to pop her balloon, confirm that there was no chance, as a joke, etc. hold water, doesn't that mark Freddie as a cad? That is a horrible thing to do to your ex-GF - especially when she is still your friend.
Do I have a ret-con (retroactive continuity fix) for the Seddie and Creddie interpretations I have for iGoodbye above?
Yes, but they are not germane to this discussion. The retcons don't count anyway, whether they be Creddie, Seddie, or neutral.