Even if you kept Sanchez, he had a season similar to last season, Ozil decided to turn up more than once every dozen games and Lacazette hit the ground running and hit 20 league goals the very best you could hope for is top 4 and a domestic cup. Arsenal are miles behind Chelsea,City and United, would take a monumental cock up from all three teams for Arsenal to do any better which just isn't going to happen.
With the way the summer has gone for both Spurs and Liverpool so far, there's an opening for 4th this season if not much happens this next month. Liverpool will suffer from having a tiny/crap squad with Europe to contend with too and their signings so far have been "meh" at best and they could be about to lose their best player. Spurs haven't signed anybody and don't really look like improving their starting 11, they'll sign squad players. You can't stay still at the top, you need to improve every year, it's been proven many times, even if you win the league with ease if you don't improve you'll find yourself miles away next season and Spurs look like doing just that at present, plus there's the whole Wembley thing which means Arsenal don't have to be that great this season to get 4th even without Sanchez. Taking the £50 million now, getting in a replacement and buying a few more players should be enough to secure 4th which is the best you can hope for with Sanchez and this way you get a fee for him.
All summer i've got why Wenger refuses to let him go this window, always maintained it's best for Arsenal to lose him on a free and get 4th place back but looking at the bigger picture now, 4th isn't looking that difficult which means Arsenal would be mental to reject £50 million when all you're achieving from it is making a relatively achievable goal a little bit easier with virtually zero chance of doing any better regardless off what your most optimistic of fans think is possible.