You touched a gentle point hosts that are advanced deal with and I guess those clients dont tell you because they are new to technology,look man to me spending money on something like whmcs is far of a luxory or something,you have to use it and put this kind of matters on your frequently asked questions ,also on your announcements,you can also put it on your tos.
To me you can leave without saying nothing meanwhile you didnt suscribe because normally hosts end vps after some post grace ,I guess the problem you depict has to do with suscriptions,If you have what it takes you shouldnt matter about they goin apart,but might worry more on other kind of things like ,maybe was I little bit rude?