It's gonna hurt them badly to see Arnold leave for absolutely nothing. 26 years old now, he's won everything with his boyhood club and won't ever get an opportunity to play for Real or Bayern if he doesn't do it now.
If he signs a new deal he's committing to being a one-club player, which although admirable, I don't think he is. Real Madrid written all over him especially, and it'll probably work out well as he's better offensively than he is defensively and they're generally not needing to defend all that much with the attacking talent they've got.
Now if at least one of Salah or van Dijk followed him out the door that would be hilarious. Last chance for a move for both of them, I can see them both going.