I dont think legends is a good counter argument that ivs are unnecessary to the variance design, as legends promotes a team switching, use multiple pokemon, catch everything you can playthrough.
Funnily, this actually applies to just about every other monster collector EXCEPT your average main series Pokemon game. You're generally encouraged to collect more monsters in order to create different teambuilds and strategies and deal with actually challenging upcoming bosses, which is something Pokemon also preaches given they put an emphasis on the PokeDex to collect more monsters and even in RBY, you could only get certain key items by catching more monsters and they play up how Gym Leaders and Elites are supposed to be a big deal but it ultimately fails in practice due to how the AI is dumber than dirt and the "bosses" not being that much stronger from your average NPC or wild encounter. They attempt to justify that under the shallow excuse of it being a game for kids, which doesn't actually work cuz kids like to be challenged and improve at games they love just like anyone else. You still get unique experiences anyway due to the skill gap between many people and their ability to improve. What may be an easy boss for some could actually go over the head of another player.