I think when you reference "producers," you really mean laborers. i.e., laborers in a factory produce goods, so you are calling them producers. In reality, laborers are actually a type of capital. They are replaceable and you buy them from a market. That's why you can get away with taxing them higher than investors. The investors provide the monetary capital necessary to buy physical capital (like machinery) and labor "capital." So you need to tax them less, or else the companies they invest in will have less monetary capital to purchase laborers, and therefore demand for laborers will diminish, resulting in job loss.
But there's a circular logic flaw in believing the two aren't related. Producers (not just laborers, think scientists, researchers, editors, engineers, managers, etc, in that they produce services and products) have some cost/value that you might term "capital" (really everything is technically capital in that sense), and investors having more money due to less taxation results in them being able to pay for more producers.
But it doesn't really work that way. Producers are typically the majority of the consumers as well, and that's money that ends up back in companies as capital that ultimately is more valuable than investor capital, since it's renewable and with a profit margin doesn't gain equity and increase in value like investor capital. But in a sense, then consumers are investors, just a different type, but they're being taxed more highly, so they can't afford to consume as much, so they invest less capital in companies.
There's a cost of living, too, that you can't ignore. If producers contribute more of their earnings to taxes and have less to live on, then the margins are smaller and they necessarily need to seek higher income to offset the taxation -- which results in their production capital becoming more expensive, which makes investor capital less potent as well.
I don't see a good argument at all for taxing people who work for their money, because all of those argument circularly apply to the working people as well, and logic would dictate that it impacts the value of capital and ultimately leads to fewer jobs. Lower producer taxation and higher taxation on investor gains may reduce the available capital (but not necessarily the amount invested) but it also reduces the cost of other forms of capital, while increasing the amount available for investment from producers. That means a much larger class of people have more room to live well and spend, versus a few living well and spending. To put it really simply: higher income taxes and lower investment return taxes results in financial segregation while the opposite tends to destroy it. It's clear, given that basic fact, why some argue for or against it. It also changes the types of investments companies need to seek, which may or may not be a bad thing.
