I think he will simply because half the population would prefer him to any likely runoff opponent. He'd obviously beat Le Pen again. Against someone like Fillon he'd also get the reluctant support of the left. Meanwhile the right would definitely back him over someone like Melenchon.
French runoffs don't always work by a strict median-voter paradigm (otherwise Mitterrand would have won 1974 but lost 1981, for example). Sometimes a candidate wins because they're the most charismatic, or there's a huge anti-incumbent backlash, or the time is right in some other way, even when it seems like they're too extreme on paper.
I agree that Le Pen and Mélenchon would both probably lose, but I'm not necessarily convinced that less toxic figures from the right or the left would. I could see two general profiles who could defeat FBM: either someone from the ""social"" wing of the right (ie mildly/performatively #populist
with a general law-and-order vibe but not mouth-foamingly xenophobic, like Xavier Bertrand), or someone who's able to consolidate both the yuppie bobo left who was disappointed by FBM and the #populist
left at the same time. That's not an easy task, of course, but it's what allowed so many ecologist mayors to win last year, so it's far from impossible.
