I wonder who from today's grid, when given a Red Bull car is able to give Max a real challenge? Charles? Lando? I think Oscar looks promising. I don't see anyone in RB's current line up being on par with Max.
Hamilton is a Mercedes driver. I don't see him even considering RB.
Whether we like it or not, age has a massive effect on driver's performance. Lewis might still have it, Alonso too but the young guns like LEC, NOR, and PIA.. They're hungry for a championship win.
I read your parent comment as being hypothetical, not tied to the driver actually having to have a contract with a team.
The only drivers who I think would trouble him are Hamilton, Norris, Piastri (maybe, he's got the pace but lacking experience), and Alonso. Leclerc is fast but too error prone.