Russell - Merc can build a race winner, he can win with it.
Norris - combination of outside chance of McLaren building a race winner, plus chance of landing a RBR/Ferrari/Merc drive.
I've discounted Sainz. I think he's a decent top tier team number 2 (or could be, if he could accept he's that, rather than thinking he's a number 1), but no more. Don't think he has the mental strength for it.
I've discounted Ocon as I don't believe Alpine build a race winner, or that he gets picked up in a top 3 team.
Piastri - I guess I would include on a similar basis to Norris, but multiplied by 0.x where x is the chances of him being a top tier driver (which I'd rate as decent, but obviously not 100% (no one is, coming in to F1)).
The rest I guess it comes down to "who gets put in at Ferrari to replace LeClerc/Sainz when the time comes to pin it on the driver for them not winning (Alonso to Vettel, then Vettel to LeClerc, style)?". That's probably not coming from their junior program, as Marklar says. I think there's a high % chance it's Norris or Piastri. Not sure who else it could be?
So ended up voting Russell, Noris and Piastri. Would say the chances* are something like 85%, 60%, 30% of it happening, respectively.
*percentages calculated by the same people as produced the AWS tyre wear graphics**
**have they stopped doing these? or have I started ignoring them so completely I just don't register them on my screen?