Of course. That is all anyone can do with respect to any counterfactual. So what? You were making a claim about the fidelity of my imagination:

"You have no clue what monolingual you would have been like."

And that's not true. I have several pretty good clues. Not only that, but what I imagine about the counterfactual me leads me to be able to formulate a testable hypothesis with respect to non-counterfactual situations, namely, that acquiring multiple languages in one's youth does afford some measurable cognitive advantages in general.

You can get a lot of leverage out of imagining things.