Give the adds a timer after which time they despawn if they wasnt killed by then, that way lower dps groups will find it easier to do dungeons / trials while high dps wont notice a difference.
If adds despawn on the death of boss they become pointless to the fight.
If adds despawn after a time limit then they become pointless to the fight.
If something is part of a fight it needs to be part of the fight. The whole encounter gets defeated or none of it.
Lower DPS isn't a real issue for most fights. All lower DPS does is force you to actually gear and face mechanics properly. It's weird but the less DPS you have the technically better player you have to be.