But the series doesn't end if people play as Imperials in this game, because you aren't playing The One Big Hero, just one person amongst many.
Helping the impeirals = helping Manimarco
Helping Manimarco = helping Molag Bal
Imperial win = Manimarco win = Molag Bal win
Molag Bal win = Tamriel absored into Coldharbor
Even if the part about equal races is true, it still makes little sense. So the Redguards and Bretons will treat the Orcs, whom they've been mass killing for centuries when there is no empire. It just doesn't add up.
Again The Redguards, Orcs, and Nords respect eachother for thier fighting porewess.
It is not unreasonable to think that two races, who have great respect for eachother, could put away thier diffrentces to fight a greater evil