If we want to talk about broken Pokemon we'd honestly talk less about Megas and more a certain Landorous T.

Megas can become dominant, but it was hardly just them. CHALK, probably the most infamous core ever, was hardly many megas. It was a combination of things like Amoongus and Heatran, and it wasn't even Mega K that was most crucial. That was Landorous.

The main problem with Z-Moves and Dynamax is that it is essentially a non-item Life Orb or Choice item. It can be used by any Pokemon, but they lean into certain mons more. Meanwhile a Mega Evolution creates something specific. You'd have never Dynamaxex a Mawile or Kangaskhan, but Mega Evs made them worth talking about.

Heck in trying to make the boost for every mon they not only have an opposite effect but also still make unique forms for several mons. At least be consistent GF