Even with the wide range of tax policies we've had over the last 70 years
...govt revenue as a % of GDP has always remained in a fairly tight range.
You have brought this up before, and Thyrsis and I, have responded. What you fail to take into account, is that higher marginal rates were doing their job. (Albeit, the progressiveness of the tax code is more important than the rate.) You say, when rates were higher and the tax code was more progressive, the wealthy were still paying roughly the same rate(6% more), because there were more deductions.
First, when the tax code was more progressive, owners/management were forced to make a choice. For example they could pay a CEO $2 million/yr, but the higher the salary, the more likely it was that that money would end up in the government's hands. The higher the salary, the higher the rate. Once we lowered the rate and broadened the base, every million paid to a CEO was capped at the top marginal rate (as low as 20% IIRC). (*Note: please check out the correlation between CEO compensation and top marginal rates, and notice the spike after the Tax Reform Act of 1986.)
You say that the rate paid by the wealthy were roughly the same. So then it's obvious, by logic and historical data, that they chose lower incomes, rather than paying the higher rates. So what did they do then, and what is done now, with the capital? Back then they took advantage of the extra deductions and spent more money. They hired more workers, they bought more equipment, or they just bought more stuff... artificially increasing aggregate demand, and wages.
And what did our government do with the taxes they collected? They built tanks and aircraft, and paid soldiers, in times of war, and built roads, schools, and bridges, in times of peace. All of these things artificially inflated demand, which in turn, artificially inflated pressure on wages.
To be a true free market capitalist you must believe that market participants act rationally. That is to say, that a person would buy product X over product Z, if product X is cheaper, and all other things are equal. Likewise, we expect the laws of supply and demand to always hold true. That's great in theory, but in the real world, people don't always act rationally. Who on this board, checked the unemployment rate, before conceiving a child? Labor isn't provided based on demand. And throughout the history of the world, there has always been way more indians, than chiefs. All those things we did back then, to make America great, was done to counter that irrational capitalistic behavior, and is blasted today as Socialism.
Now, after 40 years of a more capitalistic society, that capital lies in the hands of fewer people, and is reinvested in the form of stocks, bonds, and mortgage backed securities. So much so, and for so long, that you would admit, the better way to balance wealth distribution is to address capital gains instead of income.
We now live in a society where hundreds of thousands of people are living paycheck to paycheck. Which isn't new, but now they are buying things at 2019 prices, with 1979 wages. And every year healthcare goes up, and the electric bill goes up, and tuition goes up, and on and on. Therefore, the average American family has less discretionary income to buy things corporations produce. This was made evident, with the Great Recession. $17 trillion dollars lost, simply because investments were inflated well beyond the value of the underlying asset. If that money would have been taxed, and/or redistributed through wages, aggregate demand would have been higher and the underlying value of those assets would have been higher.
Also note that since 1980, GDP has never been over 5%. So tax revenue, in terms of real dollars, would have been significantly higher given the higher GDP rates pre 1980, even if the percentage of GDP were similar.
I was going to also write about tax rates, aggregate demand, and the velocity of money, and how they're related and how they've changed since 1980, but I'm on my 4th or 5th attempt at finishing this post. Hopefully, that explains why this post wonders so much. I'm sorry for rambling and poor grammar.
My last point: think about how many Republicans on this board, claim they were for the Trump tax cuts, but didn't support, or think tax cuts were needed at the top. Those posters are effectively saying the tax code needs to be more progressive...those socialist bastards.