the serious candidates should better wait for 2022. Better let Hollande lose and turn the page.
it's never better for parties as a whole to lose. as our dear International Elections moderator once said, "parties survive on power, thrive on power, it's always better to have it". though that obviously does not always hold true for certain superstars within the party.
if Hollande still has approvals in the mid-20s (or worse) I (as an outsider) can't imagine a viable or semi-viable politician to challenge him in the primary -- even if it's only Royal.