All of those alternative sources of money require an individual to already be quite wealthy: capital income means capital assets; gifts/inheritances means wealthy family. At that point...it honestly doesn't matter how you get your money. Over my career I have provided tax consulting and compliance services to many HNW individuals, and income taxes were never once a disincentive to working. A person rich enough to choose how they earn their income works because they choose to.

(Note: payroll taxes such as FICA, etc., actually phase out pretty quickly after $100k in earnings, so they're regressive in nature. There is the high-wage supplemental tax, but this is offset by the cap on income subject to SSI tax, so workers earnings more than $140k actually pay less in payroll tax.)

That being said, I agree that capital gains should be treated as regular income (as it was historically, pre-Reagan) and that income received via gift/inheritance should not receive a FMV cost basis.

That is probably different than the incorrect argument I've often heard, that "if I get paid more and move into a new tax bracket, I'll net less money" which shows up when someone doesn't understand the concept of taxation on marginal dollars.

I think the people who argue for higher taxes on wealthier brackets also argue for capital gains to be taxed at a similar/equal rate to income.

Right, exactly. And besides the examples you mentioned, the differing income tax brackets on married couples is also a big example of this. If only one spouse is working, and the second spouse is deciding whether to get a job, then having all of the second spouse's income taxed at a higher marginal rate from the get go can easily influence people's decisions.

Note: for married couples, the tax bracket thresholds are doubled, except for the highest (37%) bracket, which kicks in for couples making more than roughly $625k but for singles at roughly $520k.

The actual effect is that you need to actually have a huge wealth disparity between partners' earnings for the so-called marriage penalty to kick-in. Fox News notwithstanding, the overwhelming majority of married couples will not see a marriage penalty.

All true, and good points. However the marriage bonus for single earners is much greater than the bonus for double earners, so if you look at it from a certain angle there is sort of a penalty for dual earners vs sole breadwinner marriages.