I think we can agree that the Roman Empire fell as a result of some bad decisions. (Exactly which bad decisions, we may argue about). If lead really messes with your brain in subtle, dangerous ways - and it does - then it is at least plausible that some of those bad decisions would not have been taken.
But where we do have a large variety of comparable countries which phased out lead in gasoline at different times, we don't have a large variety of 1st century Mediterranean empires which phased out lead at different times. So we really can't know with any certainty.
So maybe it's more useful to focus on the more immediate causes of the fall of the Roman empire. It does depend on what we hope to get out of that sort of discussion - if it's a do and don't for modern politics, I still think "don't underestimate environmental poisons" is pretty well-justified.