The wages are absurd and I wouldn't want him back on those. Regarding him deteriorating physically I am not sure how much is actually that and just really poor man management by Mourinho. While he was with us he often was our best player by quite some distance. He has a lot of distance in him though over the last 5-7 years given how much he has played with Chile.
Regardless, for me he will always be special. During the declining years with Wenger he was a bright spot.
Alexis didn’t start playing badly when he arrived at Utd, the truth is that his underlying stats had dropped off already from the beginning of last season.
It’s not about which club he’s at, it’s about the player himself. For whatever reason, most likely the sheer amount of football he’s played, he’s at a sudden drop off. It happens. Physically he looks like he’s aged so much recently. He’d benefit from playing in a slower league, returning to Arsenal wouldn’t do anyone any good.