Either way is saying the same thing over and over again without further explanation.
There’s a quantitative relationship between risk and return. Investors on average (of invested $) will demand some particular additional return for a given increase in risk. Say there are a bunch of investment projects of a certain risk with expected returns of 6, 8 and 10%. None gteed, all equally uncertain. Say there are no taxes on investment returns. There are some investors for whom 6 is enough, for some 8 is enough, for some 10 is enough. Say in the particular market there are enough investors to fund all the 8 and 10% projects. Most projects get funded but the 6% projects don’t get funded. Now put in a investment tax of 25%. The after tax returns are now 4.5%, 6% and 7.5%. Now only the the 10% pre tax/7.5% after tax projects can attract investment. There’s less investment.
OTOH govts have to raise money and there are always competing ideas as to the ‘fair’ way to do this. So just any reduction in investment by raising taxes on capital is not necessarily a convincing argument against that tax on investment. But that is the effect at some point, taxes on capital returns reduce investment, long term productivity growth relies on investment. It’s not very controversial that a negative effects kicks in somewhere.
Singapore is now about the richest country per head whose economy isn’t based on extracting natural resources. No capital gains tax. That doesn’t make it the right policy for any country, but the idea of taxing capital returns at a lower rate is not some scam. There is an economic basis for it.
Also, zero taxes on capital returns doesn’t necessarily mean less progressive taxation. You could have a system where all savings including reinvested capital returns could be deducted from taxable income, but the remaining amount was taxed at progressive rates. Income-savings would be what was taxed, and there would not have to be any lower rates depending where that net amount came from, nor would the rate have to be as low as now for a given net amount to prevent disincentives to investing.