All well and good -- but you're making Cato's mistake: you've not accounted for human nature, and thus will end up surprised when socialism makes another leap forward.
Moreover, I think you're making a poor assumption in your own right: that the pie can only get bigger, not smaller. However, a moment's thought will tell you that a pie can get smaller. And a pie can get domestically smaller, even if it's getting bigger on a global scale.
And the way it can get smaller is if companies start paring your domestic middle-income jobs in favor of much cheaper workers elsewhere. The economic pressure from this tends to concentrate incomes at lower levels and at higher levels -- in both cases where it's more efficient to do things locally, rather than to import the services/materials. But the middle stuff can be done elsewhere.