A "brave" but obvious ending?

House Lannister ruled only one of the kingdoms. Cersei started the 3rd dynasty to claim rule on all of them, after the Targaryens and Baratheons.  Sansa being a Lannister by marriage, she can still have Lannister children with Tyrion, which is the premise for a formal reconciliation with Cersei in case her hopes for offspring fail to materialise. 

This is not unlike the founding of the Tudor dynasty, with Henry VII marrying Elisabeth of York, putting an end to the rift between north and south which had been the main divide in the Wars of the Roses. Ironically, the Tudor name is Welsh, which is another "Westerland", with sheep instead of gold.

Putting Jon on the throne would be an acknowledgement of the purely hereditary principle, which I don't believe these authors are inclined to do. Certainly not GRRM in the books, D&D will not change this. Jon will die or refuse to fight the South. He has already given up his title for Daenerys and her help, he won't be made to go to war for a crown. This is the essence of his character. He is the anti-Stannis, caring for people above rules or rights. He might pull a Diocletian, resigning or backing off from his position for the benefit of multiple successors, in our case Sansa in the North and Cersei in the South. 

Cersei's pregnancy is likely doomed. The leaks for S7 all came true but one, which saw her miscarrying at the end of the season. I suspect we'll see this happen early in S8. Set pictures showing both Cersei and Jon on the battlements of King's Landing hint at negotiations.
There is a lot of room for political arrangements mirroring actual history. Another example is Elizabeth I naming James of Scottland her successor after having executed his mother Mary Stuart. To all the Danaerys or Stark fans who have been waiting for a crushing victory of their heroes over reality, this would undoubtedly be the very bittersweet ending that was promised.

This is why I see the possibility of Cersei "adopting" Sansa and naming her heir to the Seven Kingdoms provided she continues the family name. All the "valonqar" stupid, Arya fans and other leophobes will be choking with rage 

I think they are and this is supported by the statement which is the basis for this thread: "I thought it was really brave” … “It’s very true to what ‘Thrones’ is…"
As to main characters, I see four and three are presented as protagonists supplementing each other. It makes sense that none of them comes out on top and no, the "rightful heir" is not winning. These guys will not make the apology of the hereditary principle.

I understand barely anyone wants it but I see no reason to make it the basis for any form of guarantee. It's almost an argument for the opposite.

Yes, and an anti-climax is part of the "bittersweet" deal.
The Lannisters have stayed in power all along. Keeping it going would be not only the very thing no one expects, it would be very "GoT" as well as a real life statement. Power holders succeed each other and it seldom really matters who they are. They hold the whip (and that's a good thing - the alternative is civil war or ruling gangs - electing a new clown every four or so years makes no significant difference).