Perhaps only the dead will get the sideways universe (as a reward). Those few that choose to stay on the island - Jack, for instance, will know that they are giving up some happiness (his son) for the greater good of the island.
Having said that, we can expect most of the main characters to die in the island reality so they can live out the sideways reward.
Could be Jack, could be Hugo (I can see him readily giving up his successful off island life, even without Libby, to save everyone else. The last seen shows him standing on the beach alone - or is he? (pull back and all the dead characters are standing on the beach with him)