If they haven't done anything in 5 years, in my view a reasonable amount of time, then you do get to peace them out. Just because an ally is struggling to get to them in time does not mean you should be able to peace out in time, it is just unfair on the defender, particularly since it is more a failing of the AI than a game mechanic (a human player could help fine). It would be a bit OP to allow someone to annex someone and then force their former allies to liberate them, simply assaulting castles with stacks of Mercs and forcing enemies to send their armies all the way over to you or have a large ticking warscore against them is crazy.
Incidentally though, if you do occupy your target you get some income from them and have ticking warscore against your enemies, so it has a similar effect to the Vic2 system.