There is NO WAY X-Men could tank enough at this point for Fox just to relinquish the rights. Marvel will have to buy them back, if they ever want them. So unless they would refuse to help just to try to make those right lower in value for when they buy them back, Marvel can only benefit by trying to help DOFP. Three reasons: 1) X-Men = Marvel, and Marvel can make a lot of $ themselves if X-Men rise in popularity, separate from the Box Office (the reverse of this is if Fox blows it, the X-Men are a less lucrative brand for Marvel); 2) There is talk about cross-overs between studios, which, if it ever happened, Marvel would want the best X-Men product out there to increase the cross-over benefit; and 3) Rising tides lift all ships. It is good for everyone involved, particularly Marvel, if DOFP succeeds. It will continue the success of superhero movies (good for Marvel), it will continue the success of Marvel characters (good for Marvel), and it will increase the $$ Marvel could get by licensing other characters, since Marvel could point to properties like X-Men and say "See, Fox is making bank on that, pay us for this other superhero".
Bottomline: You are wrong, IMO, Marvel has (almost) every reason to be rooting for X-Men to succeed.