The problem is not that USUM's content should have been DLC for SM, the problem is that USUM should have been the first version, it's the real full game all inside a cartridge as it should be.
Pokemon is already pretty bad with hidden content and events becoming inaccessible through normal means once they move to the following gen, we don't need to add DLC to that. They'll keep releasing unfinished games and then charge us more for the stuff that should have been there in the first place, and for how long will that even be available? What they need to take from the reaction is that they should take their time to make a full game and experience.