I think all the top teams (and the rest) are looking for the next Max or Lewis and are therefore looking at the next young driver to come through. This is why good but older drivers like Bottas, Perez etc. are moving on.
Sadly Sainz is just not in Max’s league but would never play 2nd driver at the moment. He would stand up to Max on and off track and RB don’t want that.
Sainz is a better fit with Williams and James Vowels has a long term plan. Just like Zac, James may be the one to get Williams back to the front. The 2026 regs and the cost cap give all the teams an opportunity and James has been planning for that since Day 1.
Sainz brings experience of McLaren and Ferrari and is hard working and strategic. He will be a valuable addition to Williams.