It doesn't fix rushing groups for gear or whatever, but at least this way you can get still get your skill points without having to rush the quest, saving it for when you can solo or find a friendly group.
Public dungeons already have this, where you kill a certain "group clallenge" boss and automatically get a point, so it's not impossible tech-wise.
Alternatively make dungeon quests repeatable, like the trial and solo arena ones. Then, while you still have to rush the quest the first time for the point, you can go back later on the same character and see it again at a slower pace if you want.
It's not exactly story/solo mode, but may be easier to implement.