Um its not really unheard of. Kefauver did the same in 1952, and then in 1956 entered the primaries again. Like I said, he surged ahead in the early primaries but eventually the candidate supported by the president made up the difference. Also my point is that after losing the primary, Reagan was only able to remain the face of the right because Ford lost the election. With Ford winning, he becomes the face of the right, and if he becomes unpopular, then Dole become the face of the right. He could successfully separate himself from the failures of the Ford administration while acting as an attack dog (similar to Agnew), remaining popular with both the administration supporters and the Right who in your mind would resent Ford.
Hell, IOTL, a Kennedy ran against the extremely unpopular Carter and barely won any primaries, and I don't believe that Reagan's charisma alone can account for his 1976 upset. It was the right place and the right time, and 1980 could just as easily be the wrong place and the wrong time just like it was for Ted.
(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.