I can see arguments for both sides to be honest. I keep reading its the best business decision to sell to the highest bidder, which of course it is, & as pointed out on here, if there's 1 club er's on the side of caution & is careful with regards finances it's Arsenal, & that's what is keeping this deal alive because we've cashed in in the past.
However, would it be that bad if we made a football decision first for once? I keep reading about it will be detrimental to keep an unhappy player, but then i think of what Suarez did at Liverpool, Schneiderlin did at Southampton, after they were a lot more vocal about leaving & had to stay another season, & once they got back playing with the squad it was soon forgotten & they got their move the following year. The same looks like happening this season to Naby Keita, & perhaps Van Dijk. There's also guys like Lewandowski who had to play out their last season. Countless examples.
Would it be better to take the money & reinvest just now? Perhaps. I'm not privvy to the finances mind you. Can we afford to get a Thomas Lemar for example in just now, & give him the season to bed in, where he isn't under pressure straight away as the Sanchez replacement? We do need more depth, because it's a fair drop off in quality in the 2 behind the striker once you get past Sanchez & Ozil.
We also need to think about the knock on effect for the rest of the squad. As Bring Back Swales pointed out, we do have a few who's contract is up 2018. We will defo be hoping to renew Ramsey, Ozil & Ox at least, (Mertesacker is already retiring, Cazorla we can't rely on, Gibbs & Wilshere we might look to sell). That's another aspect, who we can sell this window to bring in some funds / free up some wages. Szczesny, Ospina, Debuchy, Jenkinson, Gibbs, Chambers, Wilshere, Lucas, Giroud could all go this window. That's potentially £80M+ and ~£600k of the wage bill.
I've not even mentioned the effect keeping a top class player like Sanchez should have on the team playing wise. Basically, what i'm trying to say, is that people keep saying it's a straight business decision, take the money now or lose him for nothing next year, & what i'm trying to point out is that it's a lot more complicated a decision than that, with lots of factors coming into play. Many think with us dropping out the Champions League, & many in the squad who's future is uncertain, that should make Sanchez leaving easier. However, Wenger will want to keep some continuity at this uncertain stage, so i'd argue because of these factors it will make it more difficult for Wenger to let him leave.