Renault is spending money and recruiting heavily for their factory team. Mclaren has been sliding backwards. Renault will be nipping at the front before Mclaren; they are ahead of them now.
There are now three manufacturer teams. Red Bull now having a works status PU once again. It doesn't look like Mercedes or Ferrari are interested/have openings. He is likely looking at Renault and Red Bull. If he jumped to Renault, Red Bull would take up Sainz once again.
To keep this slightly on topic, I say either Alonso sticks around for another year or leaves for Indy and Norris slides in.