>Tax capital gains as ordinary income. It's simple and fair.
It might seem like money is money: if you have $20,000 more than a year ago, due to making capital gains on something you've just sold, you're $20,000 richer (less taxes) than if you hadn't invested. If you make an extra $20,000 in income through a salary raise (you get a raise to $80,000 from $60,000) you are also $20,000 richer (less taxes) than if you hadn't worked for, argued for, and gotten the promotion or change of jobs. And if as a side gig you sell $20,000 of software as a service as a 1-person engineer with a web app, you're also $20,000 richer (less taxes) than if you hadn't been on that market and promoted it in the right way.
And yet these three are totally different sources of wealth in every respect, and taxing them equivalently would have a totally different effect on people's behavior, as well as produce wildly different amounts of tax revenue. They are also subject to different ideas of "fairness".
The main difference is risk structure.
There's a counterparty to most salary income which guarantees it in a way.
By contrast, capital investments can go up and down, so if you want people to make capital investments it makes sense to have a lower tax rate to incentivize it.
There's a risk when you make income, but it is basically limited to the chance that your employer will go bankrupt and not pay for one month of that income. (Or similar risks if you charge someone yourself.) These risks are absolutely minimal and a whole different kind than the idea that capital investment will not appreciate (or even go to 0!)
You can feel this difference yourself quite easily: if you had $1.25M in cash how much of it would you make as a capital investment, and into what kinds of investments (how risky) if capital gains were charged at 90%? (You could keep only 10% of the gains.) I think most people would make only exceedingly unrisky investments, basically keeping it as cash. Investment would be heavily disincentivized.
On the other hand would you stay at home or work for a $1.25M/year salary if it were taxed at 90%?
Most people would still work! And there are historical examples of this (where the marginal tax rate at the highest end was 90% or more.)
So saying capital gains = income is very dangerous from the point of view of incentivizing investment (fundamental to capitalism) and also ignores the risk structure.
The only time it seems similar is in a "bubble mentality". In other words, if you think that all capital investments always appreciate. For example if you thought this way: of course it's not risky; of course it will go up; of course I would put my $1.25M net worth into bitcoin, after it goes to $20M then even if I had to let go of 90% of my gains it still makes a lot more sense than keeping it as cash. The fact is it's no different than $18.75M of income from any other source.
I hope I don't have to tell you how irrational and bubble-like this thinking would be.
I believe that in sane capital markets, capital gains and earnings from other sources are entirely different kinds of things. Investment behavior responds extremely different to these things.