There are speculations about Arsenal allowing Thomas Partey to leave this summer. Honestly I don't really have a problem with selling Partey at that amount. The problem is also selling Xhaka as well. It'll really leave their midfield lacking in experience though. In as much as I want him gone too but not on the back of losing two of their midfield generals at one go.
Remember that the midfield is at the heart of how they play. Even if the new guy comes in and hits the ground running, we can't realistically expect two new midfielders to hit it off immediately.
Those would be bad moves as midfield is what wins or loses you games.For Xhaka I think he is a great player and still not old in age which means he can sure give again to Arsenal,I don't know about Partey but as you say letting two players go without substituting them with better ones is not the best of the moves in the summer.Also let's suppose you get two other great players to replace them,these new players cannot directly produce good results for Arsenal as they will need time to know each other and the new team,thus taking more time to be harmonized as a team and this is what Arsenal should keep in mind before making any move.