Well it's true like. If you really want to nitpick you can go back 100 years when John Davies basically provided us with enough cash to get out of debt and financed Old Trafford which has been a significant source of our revenue for many years, but it is not the same as City/Chelsea. That's my opinion anyway and I answered that quite seriously.
It is impossible to separate Manchester United's premier league success from their wealth relative to the rest of the league. If City/Chelsea hadn't have come along it would have only gotten worse. Pretty much the only way to be consistently successful.