Michael presumably could have stayed in Dean without dying.
Sure, the vessel was damaged after Kaia stabbed him with the spear, but Dean is fine aside from a scar.
The Winchesters aren’t really any closer to defeating Michael than they were at the beginning of the episode, and they certainly have no idea why Michael vacated his vessel.
Still, even if the boys don’t actually have the spear, their knowledge that it exists could remain in the back of their minds moving forward.