The real reason the U.S. is so rich is that it was already the richest country in the world in 1900. The U.S. had almost 50% higher GDP per capita than western europe in 1900: https://www.researchgate.net/figure/Population-GDP-and-GDP-p.... Today, its still about 50%.
Nobody's denying that the US-created world order has been good for its partners but that doesn't mean the benefit was at the US's expense. International trade is not a zero-sum game - the lifting tide and all that.
If the U.S. obtained such a special benefit, it should have grown faster than western europe from 1950 to 1990, but it didn’t. If that growth comes from peace, not being the hegemon—as you put it, a rising tide lifts all boats—then the U.S. is disproportionately bankrolled a peace that western europe equally benefitted from.
Part of the story here is that international trade just isn’t that important to the U.S. 90% of U.S. GDP is domestic. Just 1.1% is exports to Europe.
Not necessarily; the US could have extracted that benefit by staying ahead of the rest of the world in terms of its citizens' wealth, with all the benefits this entails.
We can't know the "what-if" (would the US have become even richer by being an isolationist MAGA dreamland), but we know for a fact that the world order was created and maintained by the US, so it must have had its benefits all this time.
Did hegemony help the U.S. maintain that edge? Maybe! But I think that’s a harder claim to prove than suggested by OP. I think the direct cause of America keeping its edge in the second half of the 20th century is we have Silicon Valley. I can think of a mechanism how reserve currency status is an indirect cause: reserve currency status means the world invests in American banks, and banks then use that money to fund tech startups. But is that really what’s happening? As I said above, I’m unsure.