I don't know. Why is infinite regress "impossible?" What proof does Aquinas offer that infinite regress is not possible? If we can speak of "eternity" then we can certainly speak of "infinity".
Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false.Imagine a chain comprised ONLY of links which require the previous link in order to exist. Imagine an infinite series of these links. No matter which link you choose - on into infinity - the link it is dependent upon is itself dependent up the next link. You never get to any link that does not need a cause - by definition (all dependent links). The chain waits for some cause to ripple through, but there is none.
The chain never gets started - each one in it is dependent upon a dependent upon….
Imagine looking backwards at an infinite line of people playing tag, each waiting for their tag to tag the next. That's all you have are waiters, no one get's tagged.
If all there are are dependent existences waiting for their cause, none get caused. Nothing exists.
Since we know things exist, this explanation of existence comprised only of dependent causes must be false.