It's by far the best designed super mechanic in the series and adds just a ton to competitive battling. Mega Evolution was pretty powerful but largely amounted to giving chosen privileged Pokemon super forms that are mostly better than other Pokemon but have no hold item. The mechanic amounted to little more than adding some new Pokemon.

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 that emerges from these mechanics, and the on the fly decision making this creates is a very deep rabbit hole. Like any mechanic dynamax should certainly be open to further tweaks and improvements, but as a core mechanic, it's actually fantastic and I'd love to see it stick around. Megas can come back if they want since as long as they actually bother to balance Mega Salamence the mechanic is mostly just extra Pokemon which is kinda whatever, and yeah Z-moves can kindly stay gone forever.