Reagan would have been the nominee with or without Carter. He almost unseated a sitting President, Ford, in the primaries.
Nominee, yes, President, no.
If Carter, or any other Dem, had done even remotely well in office at that time, they would have won reelection.
Watergate still burned in many brains as being a “Republican” issue. Carter screwing up so bad made a big difference in the final outcome.