Wow. That's seriously underwhelming. So Miraak and the MQ was just an excuse to lead you to Solstheim, basically.
I'm guessing the side quests are a lot better.
Actually, the Dragonborn MQ storyline is the best of Skyrim so far. It has a much more convoluted plot.
Well according to HM, Miraak intended to use enslaving Solstheim as an excuse to lure Dovahkiin into travelling to Apocrypha to try and stop him (Miraak). Miraak also appeared to be super strong so Dovahkiin would train hard and become the peak of Dragonborn as Miraak was, all to make Dovahkiin's soul equal to his. Only with the full strength of another Dragonborn's soul could Miraak escape from Apocrypha. HM assists you in becoming the peak of Dragonborn so you can both flush the traitor, and according to him, Miraak had thoughts of this betrayal all this long.
Miraak on the other hand, believes that HM betrayed him by bringing his replacement, Dovahkiin (you), to Apocrypha. So according to Miraak, he never considered betraying HM until he saw that HM was assisting you in his downfall.
Personally, I believe Miraak's side of the story or the truth could be a mixture of both.