There's just one Marquez, if that's what they want then there's no contest really. They'd be racing against him otherwise.
Martin doesn't "deserve" (in the strictest sense) a ride in the factory team, he has to "earn" it by winning a championship, which hadn't happened so far. We've yet to see this year, but by no means is it a safe prediction.
They've all had it easy since Marquez first accident, then Honda being effectively uncompetitive. Now we can see (baring accidents) who is really the most worthy winner of the championship in rider terms.
Both Martin and Bastianinni have also had their own health issues resulting from accidents, hopefully now fully cleared for both of them to e fully complete in approaching this championship.
Martin doesn't have the bargaining power of MM, simple as that, also turning himself in a similar position to Sainz in what he's able to offer, competitive, some wins but not got in position to close out a championship, ultimately to move to another team as he views more value in himself than current achievement indicates.
The rest of this year is going to show us just who has got the tenacity, then next year with the three being in other teams will be really defining.
I still wouldn't bet against MM for championship this year