But then how developers be able to add unlockable game mechanics ?
Do you think the Tomb Raider reboot and The Last of Us have "tacked on" closely RPG related mechanics since they have skill trees ?
Honestly, skill trees sound a lot less offensive in terms of progression mechanics than having a leveling system for these types of games ... (since they aren't RPGs)
Skill trees and leveling systems have become nearly synonymous. I know there's a difference, but in my mind they're about as obnoxious and tacked on. Granted, my point isn't that an entire leveling system should be added. And it's possible that if these games tried too hard to imitate games with detailed leveling mechanics, that it would be even more annoying. But skill trees still feel uneccessary in a lot of the video games they're in (in fact I'm pretty sure a lot of video games literally call skill trees "level up" systems, since you are really just gaining experience and applying it to a tree)..
Skill trees might be inoffensive but they are offensively so. They have become so mild that their purpose has diluted. You can add progression in many ways, but as I said in the OP, I don't know all the answers. I'm not the one developing games, these designers might potentially have better ideas than me. Or they might like skill trees, who knows ...
I think for narrative focused games making it so that players get upgrades or new elements through story progression is perhaps one of the best solutions. Of course, in many of these games, there are technically new mechanics and items that you do gain through story. But I feel like a lot of developers forget that doing a small amount of things and doing all of them particularly well can be way better than adding a progression system to make the gamer feel like they're making even more progress. In other words, well timed upgrades, new mechanics being introduced, or even skill based challenges in narrative games offer more than enough variety on average. But of course, I feel like there is some ... untapped ... not yet created ... progression system. And this is just but one example of an alternative.