The US has a unique foundational culture of individualist liberalism. It is hard to understand the modern context of the right to self-government without the American precedent. Many of the things we take for granted were created by this philosophy. To compare it to other "modern democracies" is to discount the unique tradition of the US.
Compare America to America rather than expecting that the US adhere to the relatively more collectivist norms of other nations.
The Gilded Age was an even more productive era in terms of growth and upward mobility, with no income taxes levied at the Federal level. It was the progressive era that brought an end to this prosperity and ushered in the IRS, the 3rd central bank and an increasing number of foreign conflicts. Further economic interventions brought about and prolonged the great depression.
After WWII, much of the world's industrial base had been destroyed. Militaries funded by taxes destroyed productive capacity. Attributing this period of relative prosperity to the income tax would be seriously misguided. Instead you might have asked how much more successful the post-war economy would have been with lower taxes and less central planning.
https://en.wikipedia.org/wiki/Revenue_Act_of_1913
> It also established a one percent tax on income above $3,000 per year; the tax affected approximately three percent of the population.
https://taxfoundation.org/blog/independence-day-taxes-then-a...
>Taxation in the United States in 1776 was incredibly different than what it is today. There were no income taxes, no corporate taxes, and no payroll taxes. Instead, the American Colonies (and to a larger extent, the British Crown) were primarily funded by tariffs and excise taxes.
https://www.mountvernon.org/george-washington/colonial-life-...
>The average tax rate in colonial America was between 1 and 1.5%
And for that much the colonists fought a war for independence. The "modern democracies" you reach for would most likely not exist as such if not for that precedent.