He did this to prove a point to Cyke, who had had his character derailed and who everyone was treating like crap for the past while.
Something something the teenage versions of the original X-Men get stranded in the future.
Unlike normal time travel, this doesn't cause a fork in the imeline--eventually, they're going to have to go back in time in order to preserve the timeline.
Notably, this requires teen-angel to have his wings cut off and replaced with Mimic's wings becuase he got a power upgrade that turned his wings into fire. This power upgrade was an infusion of Cosmic Power so it shouldn't have worked that way but whatever.
Jean learns how to burry emotions and memories from a pair of psychic twins who have memorie and emotion manipulation powers and uses it to suppress her own and the other O-5's memories of the future until after they experience their past selves going back.
So by "go back in time to become their adult selves," I mean, "go back in time so that they can literally grow up and become the adult selves that they met on their extended trip to the future."