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.
"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."
Absolutely true. When the masses start believing that capitalism is giving them a raw deal, capitalism is in deep trouble. Eventually the socialists will accomplish change, either through the ballot box or the bayonet.
I'm not saying it's right or wrong, just saying the possibility is there and it's very real.