But level appropriate minions *aren't* a credible threat, as far as I can tell. You need 15 or so kobold minions to even have a chance at taking down *a* first level character. Same with the legion devils. You need so many to line up and try to beat on someone that it doesn't strike me as exciting. Its more like hacking through underbrush with a machete. Its a tedious chore that you are required to do to get to some place interesting.
They're a minor tactical problem (clear them or get around them to attack the actual monsters) if you mix them a reasonable proportion with real monsters, but they aren't ever a threat.
A lower level monster will hit a bit less often, but its made up for with more damage- attacks that players will have to pay attention to, not just ignore- provoking opportunity attacks from minions really doesn't matter much, because their chance to hit isn't great, and the damage is so small that it doesn't actually matter.