the people had more spendable income to put into the economy
Sigh... More Laffer optimum misconceptions.
It's true. Consider the extreme example of a flat tax rate at 99%, stifling economic development. One year, the government decides to cut the flat tax rate down to 98%. Now, you've just
doubled the income of everyone in the country! People spend significantly more than they previously did with this new excess, and the economic output of the country grows, say by 50%. That 50% growth in the economy more than offsets the 1% drop in tax revenues, and the federal budget can actually grow by cuts.
This is the part you understand well as it has been parroted for decades by pretty much every politician that ever wanted to pass a tax cut.
But it is
not a general truth. Consider the inverse. Taxes are a flat 1% on everyone. The government cuts taxes in
half one year, down to 0.5%! The government has lost HALF of its income and cannot operate unless the economic output of the country quickly doubles. The everyday tax payer.... barely notices the change and had their take-home pay increase by less than 1%. Economic output actually suffers, because despite the fact that people have (slightly) more cash, things like underfunded schools and roads hurt productivity and the government enters a downward spiral of diminishing revenues.
Reality is there is no flat tax, and it is not so simple. But that's not convenient for people who want to pretend the tax code and economy are simple things. Most of the US tax code is pretty far below the Laffer optimum already to the point that cuts actually have an outsized negative effect and don't pay for themselves. Almsot every actual expert agrees with that.
I like a tax cut as much as the next guy, but we shouldn't pretend that any budget holes won't need to somehow be plugged, either by increasing revenues elsewhere, or program cuts. Doing so leaves us in a position with no choice but to incur massive debt and argue endlessly about whose fault it is.