Governments change though, once British sovereignty is transferred to Brussels then it ain't coming back.
What I don't want is a situation where the UK government promises to cut immigration to the tens of thousands, and then doesn't, because EU rules prevent them from doing so. We should be holding our government to account, if they promise to cut net migration to the tens of thousands but fail then they should be hauled over the coals. Once Britain controls its own borders, there is no reason why net migration can't be cut to this level, so if they fail to do so it's because of their own incompetence, not because we've transferred control of our borders to Brussels who don't particular care about the costs associated with uncontrolled migration on British people.