I don’t think they have any other valid options currently, who could they sign at the moment? RBR has made Max their number 1 to such an extend that nobody could compete in that team. So the pilots that can compete with Max seem to rather stay at their own teams, hoping that they will close the gap sooner or later and actually fight with Max. The drivers that aren’t able to compete know that it is career suicide as they will compare badly against max and moving back to a back marker team might be a very cold shower after driving a race winning car. And RBR has all but ended their driver academy, so no new talent will be flowing in from that pool either (and looking at the last 3 guys sacked by Marko, who would want to).

The only possibility seems to be Ricciardo, but he might not be back up in time.