Wood does a good job covering why he got to Williams, and I would just add that there’s no Leclerc/Russell/Piastri type in the junior ranks right now. The last 2 F2 winners are Drugovich and Pourchaire, who have potential but aren’t likely to be good in Year 1 and neither has massive financial backing. On top of that, there isn’t really an obvious veteran to sign, so Williams either finds a big pay driver who’s terrible or they’re better off betting on Sargeant improving in Year 2 over starting from scratch with the currently available guys. Checo would’ve been a great get if RB let him go, because his usual routine of mixing god awful weekends with ones where he can make an alternative strategy work for points above what would be expected is a great fit for a team like Williams right now.
Tl;dr I don’t think giving Sargeant the seat last year was a very good move, but their options were limited and keeping him in the seat next year is probably their best option currently or at least just as good as the alternatives.