Newey has pretty much accomplished everything there is in F1 and he is being paid very well at RBR especially if we include the number of championship bonuses he must have earned over the year. At 65, he is likely to retire from F1 at the peak of his career with record breaking RB19, RB20 and RB21 instead of sacrificing his golden years to take on another mammoth task of trying to win another championship a new organization.
If we consider the timeline, by the time AN leave RBR, the 2026 cars will be on the grid and there will be nothing much AN can influence in the new team until there is a major regulation change. By then he will be in his 70s.
If he quit at his peak, he will be remembered as GOAT and will forever be part of RBR. Personally I would like to see AN joining FIA in the technical regulation department. For sure he will bring back the V10 engine.