So what's the basis of these extrapolations?
You presuppose that Rocky makes the same subpar running mate choice that Ford would, when the elephant in the room (no pun intended) is actually a ticket with Reagan that year. Alternately, the pressure would be high to choose a southerner capable of vying with Carter and balancing the ticket, perhaps William Brock, George Bush or Howard Baker.
You then presuppose Rocky dies before the end of his term. Now, his death in 1979 and its circumstances are famous. And there may or may not be a good basis to inform a guess that he dies in this timeline, or whether for instance the health monitoring presidents receive saves his life. But this is the sort of thing that timelines exist to flesh out.
Also, I think I pointed out when someone asked about a "Ford wins 1976" timeline that we shouldn't just immediately assume that the party in power loses 1980. Likewise, that's why we write timelines.
For all we know, Rocky in 1980 could be on the mend from his heart transplant, making the journey from his hospital bed to greet the freed Iranian hostages, and preparing to sign his landmark bipartisan healthcare reform legislation. The point is, you should plot it out and see what happens.
I mean, think of it this way: our history's 1976 presidential election is the posterchild for how history does not always augur towards the ostensibly most likely outcome. New Year's Day 1976, a very few Americans would have thought a President Carter plausible.