He becomes leass of an asset, to both City & Arsenal, at the end of the season imo.
They can get money for him now, & save his wages but he is likely to be less influential if they keep him & maybe unpopular with team mates, he walks for free in the summer & they could instead maybe bring in a fresh player now to inspire the squad.
We get an extra player now, who could be very influential when we are in 4 competitions & it is actually possible to win all of them, wheras we could sign anybody else we wanted, in the summer, it needn't be Sanchez .
Makes sense to get it done now, for both clubs & the player.