The issue with follower dungeons is not the concept, it’s the implementation and the way you enter it to be specific.
Solution to this is simple as there already is a working mechanic in-game that would prevent people from abusing it. When you want to enter “The Proving Grounds” you have to talk to an NPC and it immediately puts you in the instance so you don’t have time to run around cheating death with it. Same mechanic used to enter TW raids. So have the follower dungeons work in this way, not with LFG framework.
The concept is fine. It gives casual or new players chance to play the story and get to know the dungeons without the preassure of toxic tryhards, while not giving any substantial reward so as to not discourage actually playing dungeons as they are intended latter on.
So just add an NPC to an inn in Dornrogal or other player hub, or just have innkeeper do it, because if D’n’D and collage taught us something, its that every adventure starts in a local pub