There's a garage around the corner that tries to sell cars at ludicrously over-inflated prices. They sit there on the forecourt for months, until eventually the owner drops the price a bit. Then more months and then he drops the price a bit more. And more months etc. But then the car is older, and not worth the new lower price. So it sits there longer still.
And all that time, his capital is tied up and not earning him money. Basically the man is a fool. He's in denial, to the huge detriment of his business. He fails to grasp the nettle and to realise that his asset is only worth the market price and he should sell it for the market price and move on.
The way Arsenal should be looking at this is:
To sign a top, top striker on a 5-year contract is perhaps going to cost us £100m in the current market. If we sell Sanchez for £50m, it's going to cost us £50m net. If we wait a year, we'll have to let him go for nothing and then we'll have to fork out perhaps £120m for a replacement, because the prices are definitely on the up. Therefore we should man up and buy a top quality replacement for Sanchez right now. Give the fans something to actually cheer about, instead for forcing a player who doesn't want to be there to play for you.