I mean, I feel like the thing that made megas good is that each pokemon that got a mega evolution got its own individualized one
but that's also the problem, because they're not going through and giving every pokemon its own custom powered-up form? even aside from the work that would need to be put in to design them, it'd nearly double the number of different creatures and their characteristics to keep track of, since there's not really a consistent rule or procedure for what mega evolving does to the base form? Z-moves and dynamax are mechanics any pokemon can use and access, even if some mons *also* got individualized fancy ones, and aside from those fancy ones they mostly had simple and consistent transformations they applied.
megas felt pretty centralizing, in the design of teams for characters who were supposed to have one or in more competitive play or in, like, if you just don't happen to have any of your favorites have megas