Is this because they failed to win for three years in a row? Like are they trying to do an overhaul of some good and some not so good players in order to get a few great ones instead and make a bigger change? These players are all "decent" players, they can fetch a total of 100+ million easily, but will they be able to sell all of them at the same summer window?
We need to remember (and obviously they need to remember) this isn't football manager. You can't just put players on sale and hope they will get sold, even there you have hard time getting offers most of the time, I remember trying to give players away for free, and they still didn't get sold because of their high salary. So Arsenal have a good idea about this, but I am not convinced that they will be able to do it.
Yeah this must be for the reason they want to play bigger in the Premier League. I would also get bored of being the 2nd team all the time if I were them.

I also wonder whether they will manage to sell all these players. At least selling the most useless ones would be okay anyway...

I don't know how true this is... But if it is then it would be the reasonable thing to do for Sporting. Because there is no way they are selling him for a price like 85 million pounds (~100 million euros).
His market value is 75 million euros according to Transfermarkt. They should be more reasonable with the price. He is a great striker but expecting 100 million for him is too exaggerated.
