Actually it doesn't as they don't own the rights, what Fox have is essentially a longterm lease on the rights, and as long as they uphold the terms of the lease they will hold onto the rights indefinitely. Fox can't do anything with the rights, only what is allowed by the terms of the rights contract. Why this matters is that while Deadpool and the X-Men have links in the comics, if the contracts don't recognise those links and allow crossovers (and when most of these contracts were made crossovers weren't something that was thought of) then any crossovers between Deadpool and the X-Men would would be outside the terms of the contracts and require the approval of Marvel. If those sorts of clauses didn't exist then Fox could put the X-Men in Alien or Predator films and things like that. You have to remember that different film rights have different terms and give the rights owner different percentages of the profits. Once you start mixing film rights you have questions of which contract counts on things like percentage of the profits, and if there are reversion clauses (like the the Marvel rights) do the films count towards keeping the rights active.
Thats is why I'm interested to see what happens as it will give more information on what sort of arrangement allowed Deadpool to appear in XMO:W.
There seems to be three things that could have happened, Deadpool got absorbed into the X-Men rights, the appearance was a onetime arrangement, or a longterm deal was made to allow them to crossover. I would rule out the first as I don't see Marvel wanting to give more to the X-Men film rights, it has always been known that Marvel didn't know what they were doing when the X-Men rights deal was done and allowed themselves to be screwed. So you would think that if Fox tried to add more characters to the X-Men rights Marvel wouldn't allow it without some sort of renegotiation on the terms, and I can't see Fox being willing to do that for Deadpool.
So that leaves us with the XMO:W appearance being some sort of crossover deal, the question is if it was a one time deal or something more substantial. If it was a onetime deal then I think we can be pretty sure that the Deadpool will have no X-Men links as the relationship between Marvel and Fox is very poor these days.