You could say that about other games as well - GoW? The story was completed after 3rd, Kratos has found his peace. TLoU? Didn't ever need a sequel at all, it was perfect as it was. And so on. As long as those sequels sell, and sell a ton, Sony will continue to make them. UC4 sold close to 20M copies now, a sequel would be a guaranteed smash hit, and Sony's best chance for a MP game with decent community (DLC/MTX). It wouldn't have to surround the characters from previous entries, it could've been a sort of a soft reboot, with different cast and their own storyline, like young Sully, more Elena, Sam even, or someone totally new, but everything kept within the same (successful) formula, gameplay, and story about treasure hunting and exploration.
Fatigue is something that is going to be different for different people, and this is a hard post for me to articulate.
In my opinion, Sony have always exceled because they don’t allow themselves to get stuck in the ‘franchise rut’ to chase profits. They aren’t scared to cut the cord early, before something passes its sell by date, which allows them to create their next fresh and interesting series. By your logic, if Killzone Shadow Fall had sold 20 million copies (I know), we may never had had Horizon Zero Dawn, which would have been a great shame. Same for Ghost of Tsushima, would that have been made if InFamous SS sold 10 million copies or would we currently be looking forward to InFamous 5 or 6?
I agree with you that the story for God of War was told after God of War 3. The following entries plummeted in sales, which suggests that other people agreed. God of War 3 sold over 5 million copies, Ghost of Sparta 1.2 million (the previous PSP entry sold 3.2 million) Ascension sold a paltry 500,000 copies. Santa Monica made a herculean effort to revamp the franchise with God of War 2018; changing the core game play, changing the player’s perspective, changing Kratos himself and shifting the focus from non-stop hack and slash to introduce a compelling narrative. This is an absolute outlier.
I can only speak for myself, but I have been hugely invested in Nate, Elena and Sully. For me, they ARE Uncharted. The chemistry between them, the dynamics – it is lightning in a bottle, unique. If ND do make a new game with Nate’s daughter as the protagonist, I believe it will be extremely difficult for them to make such a compelling story (as the original 1-4). Impossible? No, but unlikely in my opinion. And why slap the Uncharted title on the game if it’s a new host of characters? Why not start a brand new light hearted adventure series free from the lore and restraints of Uncharted? I know the answer is ‘because Uncharted has brand recognition so will sell more’ but that does nothing for me. And it wouldn’t be a one off, it’d end up being the start of a brand new trilogy which would commit ND for the next 15 years.
Some games in other genres (racing, fighting, sports, action platformer) might get away with having 5+ entries, in my opinion this is because they are light on stories therefore light on emotional investment. However with narrative driven games it is more difficult in my opinion. The first 3 Gears of War games were hugely successful; Judgement, 4 and 5 are beating a dead horse story wise. Ori is another one that should stay at two entries in my opinion, Moon have said they are satisfied with the story and it has a satisfying conclusion. And in my opinion, the worst mistake Microsoft ever made is allowing Bungie to become independent so they could go and make Destiny, whilst Microsoft continued with Halo. The right decision would have been to discontinue Halo, or put it on hiatus, after Halo 1-3 and Reach. It would have took balls, but they should have fully backed their first party studio to make Destiny and put their full weight behind it. In hindsight, I believe Microsoft would have been in a better position if they had. I suppose I’m bringing in Xbox to this because I feel that Xbox during the 360 era cut back on risks in order to chase the ‘guaranteed successes’ and in the end the ‘guaranteed successes’ weren’t guaranteed once the very fine line was crossed and they became tired franchises.