Dynamax is by far the best designed super mechanic in the series and adds just a ton to competitive battling. The fact that any Pokemon can do it without a required pre-game set-up allows it to be a much more core part of gameplay and less just a gimmick, and it ends up being a resource you watch managed in a lot of clever ways.

Mega evolution isn't a particularly bad mechanic, but it's basically just "this is a particularly good Pokemon with no hold item". Megas didn't really influence the flow of gameplay; they were just a set of Pokemon you could use for the most part. It's just a lot less interesting than Dynamax.

Now think about how Dynamax actually plays. You can max anything at any time, but you only can max once per battle, the max only lasts three turns, and it ends with no compensation to the user if you switch out the maxed Pokemon. Max moves are strong, but they aren't strong enough to be indefensible by bulky Pokemon, are predictable in that their types and strengths match the natural coverage of the base Pokemon, and they have some other big limits. They can only single target, and all non-offensive moves turn into Max Guard which is a useful move but really limits your immediate versatility. The added effects are mostly good but they open the door to a lot of counterplay (Defiant Pokemon feast on the stat drops, Burning Jealousy punishes the stat boosts really hard, sometimes you pin yourself into changing the terrain or weather in an unfavorable way). It's also really relevant that max Pokemon lose the ability to take advantage of choice items, and while this can add flexibility, it also can add some significant downside (choice items offer huge stat bonuses that often are critical to your success). There's just so much gameplay emerging from these mechanics, and the on the fly decision making this creates is a very deep rabbit hole.

As a core mechanic, Dynamax is fantastic and I'd love to see it stick around.