Are people happy with the prospect of paying him 400k a week? He is a good player but he is not worth that or close to it.
I look at it like this: Our team is lacking a world class attacker. It has a lot of young players, nobody in their prime, nobody who is a world class talent who can do everything on his own, and reliable to produce week in week out. Arsenal have Sanchez (and Ozil). Chelsea have Hazard. City have Aguero, De Bruyne and Silva (and then Sterling and Sane performing brilliantly this season). Liverpool have Mane and Salah, and had Coutinho. United have Pogba, then young players who haven't quite made that step up yet. They need that world class player to elevate them.
So then if you look around the world in who we could sign to be that attacker for us... There isn't anyone. They'll cost 100m at least. Rumors that Dybala/Griezmann might be on the move, so they'd be potential world class attacker who are moving. Griezmann is probably going to Barca, Dybala will probably stay for the time being. But it doesn't come along too often. Sanchez is one of the very few around who are available that we really should do whatever we can to sign. Him being 29 is of course a negative, but he just turned 29, and is in terrific shape and has a Ronaldo type obsession to being physically a beast. So I think he's got another 3 full seasons after this half season of still being quality. The alternative to not getting him is hope in the slight chance that we manage to bring in Dybala (unlikely), or just hope Martial takes the step up and turns into a world class player without having someone else there with him to help him reach that level, and the others all improve. Obviously not the worst scenario, but we are definitely lacking that one star in attack and without that, we'll be hoping for gradual improvement.
So in that case, yeah I think we should do all we can to sign him.