Everyone in the story seems to take years long summers and winters for granted but there must be a reason Martin included them in the story. I suspect there is some sort of balance issue that has to be resolved. Uniting Jon (ice) and Daenerys (fire) will probably be part of it but I think his story is going to be deeper than the return of dragons and who rules the seven kingdoms. We see both good and bad in the fire side and in the ice side so I don't see one side winning of the the other something more along the lines of the old Chaos and Order stories that were so popular a few decades ago.