Observed reality suggests that all existence = caused. Which is the basis for Aquinas' premise. If that premise is true, then the opposite "all existence ≠ caused" is not true. If you want to play by reason/logic rules, this is why Aquinas' argument fails: the uncaused first cause cannot be said to exist if all that exists is caused.
My apologies I was thinking creation rather than existence. Aquinas doesn’t argue that existence=caused. He argues that in the sense world all we observe are intermediate and end causes… the first cause must exist because these exist.