First off, Mamdani does strike me as the kind of politician who is genuinely happy in his role and not someone who's treating it as a stepping stone to a higher office.

Secondly... the track record of NYC mayors' post-mayoral careers is abysmal. Not one mayor appears to have successfully held elected office after their mayorship. There's one failed bid for NY governor (unexpected primary loss to the senior Cuomo), and four failed bids for president, the most successful of which being Bloomberg's 4th place showing in the 2020 Democratic primary. So as a springboard to higher office, NYC mayor is absolute garbage.