The Cosmological argument is developed around a distinction between that which has necessary existence and that which is contingent. A thing that has necessary existence must exist in all possible worlds, whereas a thing that is contingent may go out of existence. The method Aquinas uses is to set up the contrary position, then prove it to be wrong. Therefore, the cosmological argument begins by accepting the premise that all things are contingent. If all things are contingent, i.e., if all things can go out of existence and do not necessarily exist, then there must be a time where all things go out of existence.
Real possibilities show up.
It is a real possibility that if everything is contingent, everything could go out of existence at once, given that time is infinite at any point, such as now. Yet, that could be because everything pops out of existence, then back into existence. Once something ceases to exist, it cannot suddenly exist again. Our sense perceptions and experience tell us that something exists, something is there, therefore everything has not ever gone out of existence. Therefore, all things cannot be contingent.