For me, it’s an uncanny valley thing. If the only thing they change is skin colour or gender, and it’s also relevant to the plot, it’s too close to the original for me to enjoy it as a new thing, but too far to be enjoyed as a new thing. It fucks with my suspense of disbelief, since I’m supposed to know stuff from other movies, but not all the things, so I’m fucked if I pretend that it’s just another episode of the same thing, or it’s a completely different and new thing equally.

That said, Velma is different enough that it’s “past the valley” for me, it’s so far from the original that it could be enjoyed as its own thing, if it didn’t fall flat for other reasons.