I think the point of not challenging for the PL or the CL is key. If I was Sanchez, being on the top of my game, and 28, I would ask myself where will I have the biggest chance of winning PL and CL within the next 2-3 years. Is it Arsenal, Bayern or City? Now we can argue the whole day about the answer to that question, but I think City is the best guess. With all due respect I do not see Arsenal under the current management change significantly from what they have been the last years, so it will be FA cups at the most in the next 2 years. Bayern is going into a generation change with major players coming to an end of their careers (Lahm, Alonso stopped already, Robben, Ribery next), so it is reasonable to think they might not be able to win the CL anytime soon. Lastly there is City with Pep and all the money to invest in the team ...