Laughing Sanchez > Entire Arsenal's lineup.
The impact could be even similar to when United sold Ronaldo to Madrid... and Ronnie was thinking about Madrid every year while destroying team after team as a Red Devil. You just don't give your best player in years, especially on a deadline day when you know, you won't be able to replace him.
They are going full backwards with this and if they'll try get another player of that caliber, Stan Kroenke will have pay over 120m and we know that will never happen.
Completely different situations. Ronaldo was still under contract at United and thus they had a degree of power, Sanchez won't be in 9 months time and therefore knows that whatever he does this season, he'll have a tonne of suitors in the summer and his pick of clubs. It's also been said that United & Ronaldo had an agreement where he could go to Real the next summer if they had a reasonable bid, was it not? That's a club and player coming to a mutually beneficial deal for both parties, and Ronaldo has always had the upmost respect for United even since leaving and therefore put in maximum effort.
Arsenal refusing to deal Sanchez when he clearly wants to go and they can get reasonable money for a player with 9 months left on his contract is not both parties working together to find a mutually beneficial deal, it's Arsenal doing what they want and ignoring the wishes of a player who's dragged their sorry club to some degree of 'success' the last couple of years when most around him have been hopeless.
Ronaldo - for all his attitude problems over the years - has always had an unmatched and unwaivering conviction to be the best which is precisely why he's as good as he is, and thus I don't think I've ever seen him down tools the way Sanchez has appeared to at points last year when he wanted out, or appear to take joy in his team being beaten, so we're talking about different players as well. Just because Sanchez runs around a bit and puts in a shift when he's happy where he is and enjoying his football, doesn't mean it's 'in his nature' to put in 110% all the time when he's not so happy, he has no reason to, infact knowing that he'll have a host of elite clubs after him in the summer no matter what, he's more likely to put in a half-arsed shift all season just to piss off those at Arsenal who've forced him to stay.
Selling Sanchez obviously isn't ideal, but I think weighing up the situation, when he clearly doesn't want to stay, there's a risk on how much they're actually going to get out of him, they can probably get £60m for him (and whilst it's not ideal either selling him to City a Prem rival, he'll probably just end up going there in 9 months time anyway and Arsenal are not going to be in the title race this season regardless), then surely it's better to cut your losses and take that and get somebody like Lemar in (even if it's £80-90m) and build from there?