People like to talk about exceeding their potential, but that’s a little bit like giving 110%. Logically speaking, once anything realizes its full potential, it can not exceed it. So when one speaks of someone exceeding their potential, they’re implying a temporal component, in essence suggesting that the previous judgement of potential was in fact an underestimation. Or else they are referring to some 3rd party judgment as to the bounds of their own abilities.

So in judging whether America has “already well exceeded its potential” my answer is that this is only true if we’re talking about the judgement of past pessimists.

Your characterization is especially peculiar given that you lean liberal. A basic part of the liberal litany is that we can and must do much better, especially when it comes to social justice, equality of opportunity, and so on and so forth. So if you believe that these things are both necessary and possible, then you MUST concurrently believe that America has NOT reached its potential.