Mind you, people probably don't think of your standard high earner they they think of an income cap. They think of people who make four (or even five) digits an hour, a rate that maybe high end lawyers can match. Maybe.
CEOs of large companies can easily make that much, often not even tied to performance but contractually guaranteed. The super-rich make that much simply by existing.
Basically, if your labor (or mere existence) isn't even worth 1000 bucks an hour to your clients you're a peasant like the rest of us and an income cap is probably never going to be relevant to you.
I agree that going for wages in the traditional sense doesn't catch many of the most relevant income streams. However, I think that a "maximum wage" makes sense as a theoretical construct used to create a sensible income tax scheme.
Essentially, tax brackets and rates could be defined in relation to the median income. Go too far above that (hitting the "maximum wage") and your tax rate rapidly increases, maybe even going as high as 90%. Of course this would have to cover all sorts of income, not just plain money.
This scheme would effectively box people into a certain band of acceptable wealth and would create an incentive to raise wages – after all, if the average worker makes more, so can the most wealthy.
(Also, full agreement on needing to talk about better labor protections. American labor law is really lax.)