The reason why they have not dismissed Perez is that they have no replacement that they can insert without leaving VCARB RB without a driver.
None of their junior drivers are ready for a move up to F1, and Daniel Ricciardo went back to Perth after his last race for VCARB and I suspect he is currently professionally unavailable, after Red Bull (as they always seem to do) completely mishandled his replacement at VCARB by Liam Lawson.
I suspect that they would like to try Franco Colapinto, but he is committed to Williams, and Williams are going to want a large bag of $$$ just to make him available for next season.
Borrowing a reserve driver from another team risks IP information transfer issues.