Well, first off, I'd just like to point out that this is a very funny thing to say about *this* series, of all things.
They *did* just totally rewrite a character of God of War, to absolute great success, and he's the goddamn protagonist. They just decided "Hey, what if we had a character that was kratos and had his background, but acted different, looked different, dressed different, talked different, used different weapons, different combat system, different world, moved differently, talked with different characters that are almost 100% unrelated to his past, and only kept his background as maybe a thematic element" and it worked so well that people now can't even imagine things differently. So, barring anything else, assuming worst case scenerio and we do have to change atreus to make him fit, that's clearly more than possible.
I don't think people have a proper appreciation just how shocking little current day kratos or even Kratos at the start of the 2018 game is from anything that came before him. He *is* a totally different character in literally every way. They did totally rewrite him. And it *worked*. Why are we pretending this is something devs can't do?
To be honest, I think the people you're talking about are a minority. Not people who prefer Kratos to Atreus, but people who hate Atreus so much that they can't stand playing him. The only time I've heard anything like that is with regards to the yak riding section, and I think we can obviously agree that was more due to the boring game design than anything intrinsic to Atreus as a character. But I think most people regarded Atreus sections as...fine? Definitely a step down from his daddy, but no one was like saying that they're a total failure or anything. They were engaged enough by his story of trying to explore his heritage, of trying to work around the prophecy, of trying to play a trick over Odin and get the better of him, and enjoyed his side of the drama with his father.
If anything, it's a major flaw of Ragnarok that they short changed Atreus so much. They never gave him his own interesting levels, his own enemies, no diversity of weapons, no unique climactic setpieces (aside from one minor one). Yeah, I'm not surprised that people don't think he can fill Kratos shoes if they think a game featuring him would be just what Ragnarok offered again. In this (and many other ways), Ragnarok would have benefited if it had been more like TLoU2 where the two protagonists truly have an equal claim to the story, with their own fully realized campaigns. As is, all Atreus has to work with Kratos' sloppy seconds.
Imagine if the game was reversed for a moment. Imagine if Ragnarok was 80% Atreus' game with Kratos getting 20% of the gameplay, if Atreus got all the weapon and combat upgrades and Kratos had a handful paired down, simplistic combat sections with reused and largely uninteresting levels. Do you think people would call Kratos the better character of that game then? So why are we acting like this is something intrinsic to the characters and not just the development resources they get. The only thing separating him from Atreus is that he is the beneficiary of having a number of well crafted games utterly devoted to him while Atreus got to play a secondary character (not co-protagonist) in one and a side protagonist (at best) in the other.
Again, all we've seen of her is maybe a 15 minute gameplay section when she's 12. Do you really think there is no room to fill in any kind of detail? They could add literally anything, before or after the sliver we got in the epilogue of Uncharted 4.
And I honestly think you're making out Drake's background to be more than it actually is. In Uncharted 1, there is no street urchin background, he's just after the money and happens to have a background and history while believing that he's Francis drakes decendant, but he's openly just there to get the cash. In uncharted 2, unless I'm remembering wrong, his background doesn't come into it at all, he's just salty that Flynn fucked him over and wants the treasure because, again, moneyz. Then in Uncharted 3, the street urchin thing is part of it, but it's at the very least equally about his attachment to Sully and choosing to value family over the thrill of adventure (which doesn't factor into his legacy at all). And then in Uncharted 4, legacy is barely a touchstone. He's doing it because he wants to help his family (sam) but because he's miserable doing his boring 9-5 office job that has him digging up crap out of the ocean.
Barring Uncharted 1 where he's purely monetarily oriented and actually wants to GTFO as soon as possible, Drake has been characterized as doing this because it's what he lives for. Yes, the legacy part is a factor, but it's a minor touch, not the cornerstone of what makes drake appealing and we know it because it's only a factor in one game.
So to argue that we can't do Cassie because she lacks this factor (and then arguing that there is no other form this legacy motivation can take - for some reason, Cassie can't just want to live up to the legend that her mom and dad have set in their field?), she isn't a valid candidate as a protagonist? That makes no sense to me.
