Sainz and RB/Max is not a good combo in my opinion. Sainz has ambitions, dreams.

Just like Checo, dreaming and fantasising that they also could become world champion! They are wrong and delusional if they have that in the RB 2nd seat. What RB needs is a safe pair of experienced hands that will be glad to be driving F1. No lofty ambitions, just the realisation what they are here for: help clinch constructors title and support Max in races so he is not sandwiched between competitors.
I.e. they need Hulkenberg, Ricciardo, Magnussen. That kind of driver.
 
I will give RB the benefit of the doubt though, they must have been baffled by the attitude and misplaced dreams of Checo and his entourage (nay, the entire population of Mexico). They gave an older reasonably good driver a lifeline back into F1 and then he gets uppity? And then underperforms like a rookie?

Boot Checo today if not tomorrow.