Eh. If we're forced to sell due to him signing a new contract, it will not be to a domestic rival. It'll hurt us badly, but it won't help anyone else here - that's acceptable.
I can see it from his perspective - he's entering his prime, and he's probably worth a 150-200k contract at one of the megaclubs that would see him through to the end of his career. That release clause Di Marzio's chatting absolute brick about has been debunked before - it does exist, but it can only be activated in very limited circumstances (if we choose to take up the one year option on Alderweireld's existing deal in 2019, I believe - this comes from Krisztof Terreur, who has been 100% accurate about our Belgian contingent and the Prem's Belgian players more broadly over the past couple of years). However, that doesn't preclude the possibility of us selling him this summer or in 2018 if he doesn't sign a new deal.
It will hurt whenever it happens, no two ways about it. Probably a lot, because he's the reason our defense is the way it is - not Poch, not Dier or Verts, not Lloris, not the full backs, not the Dembele-Wanyama screen ahead of them, him and him alone. That's just an established fact at this point. But we're not going to go through this entire 2017-2022 period without losing some of our established players, so it wouldn't be overly shocking either.