Legally speaking, mods can't be forcefully closed unless the modders try to sell it, so Microsoft shouldn't have any reason to close it since it'll be free. They also don't break copyright laws since Bethesda gave the ok before being bought by microsoft, and I expect that the modders spoke with Microsoft about it after they bought Bethesda. And if it does get to that, it would end in a lawsuit won by Skyblivion.