IVs benefit the game in no way, shape or form, whether you're a casual or competitive player. Hell, if anything in competitive it's annoying when your Latios loses that crucial speed tie because of HP Fire. In addition, it completely locks off and turns people away from Competitive because in-game breeding is such an annoying, tedious, long, arduous task. If they were taken out of the game, nothing of value would be lost.
I'm actually very okay with Natures and EVs because they allow for a great deal of customization and experimentation. With IVs it's "either have the best or gtfo" with no variation; but with EVs and Natures, you're forced to choose what your 'mon does best. Do you want to go Bold Max defence to make them a bulky monster, or do you want them to be Timid max special attack to make them a special sweeper? While this obviously will depend a lot on base stats and other factors, this still allows you to come up with new interesting combinations you wouldn't think to try on your Pokémon if they didn't exist.
I'm actually very okay with Natures and EVs because they allow for a great deal of customization and experimentation. With IVs it's "either have the best or gtfo" with no variation; but with EVs and Natures, you're forced to choose what your 'mon does best. Do you want to go Bold Max defence to make them a bulky monster, or do you want them to be Timid max special attack to make them a special sweeper? While this obviously will depend a lot on base stats and other factors, this still allows you to come up with new interesting combinations you wouldn't think to try on your Pokémon if they didn't exist.