Never did an incumbent president participate in a primary debate. Do you think there's a chance this will ever or in the median future happen? To be fair, most presidents seeking reelection didn't face serious competition. In the age of TV debates, only Ford and Bush 41.
Uh...the primary challenge to Carter was more serious than the one to Bush 41. In fact, I would say Bush 41 is the least likely of the three to have engaged in a primary debate as Buchanan didn't have any shot at the nomination or even any of winning a single primary.
I think an incumbent president facing a primary challenge may well have to debate...there's just more pressure than there used to be, but then again, an incumbent president would actually have to face a serious primary challenge...something that hasn't happened in awhile.