(1) Kennedy won the primaries or caucuses in MA, CT, NY, PA, VT, MI, DC, CA, NJ, NM, RI and SD. I wouldn't call that "hardly any" especially since those include some very populous states. https://en.wikipedia.org/wiki/1980_Democratic_Party_presidential_primaries
(2) Kennedy was under severe disadvantages--not only the "character issue" (mostly Chappaquiddick) and the disastrous Roger Mudd interview but the fact that in the early part of the primary season Carter was enjoying a temporary resurgence of popularity due to the inclination to rally behind him in the first months of the hostage crisis.
So if anything Carter in 1980 shows the vulnerability that year of an unpopular president (or his designated successor) to a challenger more in tune with the mood of the party.
And Reagan is still going to be the face of the Right. The fact that conservatives reluctantly supported Ford over Carter doesn't mean they will view Ford as one of them ideologically--especially if he can't prevent the Iranian revolution, the Soviet takeover of Afghanistan, etc. Nor will they so regard Dole, who will have to defend Ford and lose support on the Right the way Humphrey's defense of LBJ cost him support on the Left.
Also, remember that Reagan's showing against Ford in 1976 was under economic conditions considerably better for the incumbent than those probably facing Ford in 1980.
The most likely course of events is that Reagan beats Dole, but loses decisively in the general election. After 1980 Reagan never pursues public office again, although he'd probably continue to campaign for Republican candidates and speak out on important issues.
As for Dole, he might be able to make a political comeback by being elected Governor (a return to the Senate is unlikely given Kansas's heavily Republican lean, which would give Dole few opportunities for a second Congressional career) and running for President again later.