Every time an incumbent is challenged by a member of their own party that party loses the general election. I do not think that would be a good outcome in 2028.

Furthermore, I think it is likely that if tfg loses (please god) R's will regroup around the non-MAGA wing. The Cheneys, Kinziger et al will leave off supporting Harris in a heartbeat and D's will have a serious problem even maintaining victory as incumbent. If she were to face a challenge from inside the party too there is no way she will be president on Jan 21, 2029.