Allow me to explain why Dynamax is the superior mechanic. All 3 mechanics require the Trainer to hold an item, but only Dynamaxing doesn't require the Pokemon to hold an items. Because of this, players don't need to slap a Z crystal/mega stone on every Pokemon in their party just in case they need to mega evolve or use a z move. Instead, players can Dynamax Pokemon while ALSO having any other items they want equipped to their Pokemon. Dynamaxing succeeds where its predecessors failed. You can Dynamax while still having a party full of whichever items you want, which allows for more strategies and more varied/interesting battles.