Read any interview with DnD about the Red Wedding and you should see that was all they really wanted to make as they knew it would make for shocking TV.
Your comparisons of Jon and Frodo show how much you are stretching to make sense of the awful narrative decisions in the later seasons. If you didn't consider Jon as Frodo originally, who did you consider him as?
The ending of a story should not make you reevaluate the narrative and arcs that have been building over multiple series and novels.
While I believe they have hit the broad strokes of GRRMs outline (mainly mad Queen, as that is telegraphed a lot more in the books) they went off the deep end with some of their adaptations of storylines, mainly Jon. You simply do not spend 7 seasons building a character to make him a shadow of that character at the end. Arya's downing the NK was definitely their final attempt at shock and awe which made the show so famous in the early years.
If you don't think the ending of a story encourages reevaluation then I think we just look at storytelling in very different ways. For this reason alone I can't see us ever meeting in the middle on Jon, but I'll give it a go anyway. For the longest time I thought Jon
was Aragorn -- the secret king, the prodigal son, the just ruler who would repair the world and live happily ever after. Find my posts about him from before season 8 if you don't believe me. Until the point of his death, I think Jon was just that. He joined the Night's Watch of his own volition, he lead the mission beyond the Wall to avenge Lord Commander Mormont, he was willing to sacrifice himself to assassinate Mance Rayder and then to retrieve the wildlings from Hardhome. As Sam said during the Night's Watch election, he was "the man we turned to when the night was darkest". A hero, a proper hearthrob, a king.
But then he was killed. All that nobility and leadership just sent him to his grave. After he came back, he was different. He was essentially dragged through everything he did by other people. First by Melisandre who brought him back in the first place, then by Sansa who convinced him to retake Winterfell even when he said he was "tired of fighting", then the Northmen who crowned him king, then by Daenerys who convinced him to bend the knee, and then by Tyrion who told him he had to kill Daenerys. For a lot of his journey after his resurrection, he was passive and living in denial. Tired of fighting and losing, tired of finding no joy in his talents, tired of power destroying everything good about the world. He gave up his role as Lord Commander, he gave up his title of King in the North within weeks of being crowned, and he continued to refuse the Iron Throne even when pushed by everyone around him. He didn't want the throne.
If Jon
had taken the throne at the end, it would have not only received backlash for being incredibly unimaginative and a contradiction of everyone's favourite meme ("Ah dun wahn it"), but it would have endorsed the type of ending that GRRM didn't want. If you want to bring the books into this then let's do that: GRRM wrote A Song of Ice and Fire as something of a response to the end of Lord of the Rings, which suggested that a good king automatically equals a happy and prosperous land. GRRM wrote the books to argue that history has always taught us the complete opposite. If Game of Thrones ends with Jon on the throne as a good king who defeated the zombie army, ended all evil, and ruled forever and ever, then it would have basically been a rip-off of a thousand other fantasy stories and undermined the intentions of the story's creator. Jon "being built up across seven seasons", as you put it, is just the story providing you with the rug that it will eventually pull from under your feet.
Jon being built up to be king only to end up ruling nothing has the same intentions as Ned being built up to be the hero only to be killed, or Robb being built up as the new hero only to be killed, or Daenerys being built up to rule fairly only to tragically fail, or Oberyn being built up to be the hero only to be killed, or Margaery being built up to defeat Cersei only to be killed, etc. Some of those stories were told with more grace, nuance, and slight of hand than others, but the principles were all the same. That's the kind of story A Song of Ice and Fire & Game of Thrones both were. That's not to say more traditional heroes don't have a place in modern storytelling (the popularity of the Marvel Cinematic Universe suggests they do), and that's not to say anti-heroes are the only worthy form of protagonist these days (because that also just isn't true). But ASOIAF and GOT just told their own types of stories, one in which characters like Jon don't end up ruling the world forever and ever.
I've read many interviews with D&D (as you can imagine) and I always interpreted their comments about the Red Wedding as them simply being anxious to deliver it, not to just reach that one shock moment and then mentally check out. In fact, the best season (four), best episode ('The Winds of Winter'), and best scene (Hardhome) were all still to come. I'm not sure the show would have sustained such a level of quality and popularity during seasons 4-6 if they'd simply drifted through it without giving a shit. As I said, the show changed its priorities in later seasons, and I think D&D lost confidence in their ability to scrutinise the subtext and nuance underneath the plot they were crafting. I think the pacing of the last two seasons was affected by them a) not slowing down to let the characters reflect and take stock, b) thinking massive battle set-pieces were what the viewers wanted most. But I think there's a difference between them having shortcomings as original storytellers and them simply not being interested anymore.
