> There is plenty of money, we could tax something like 5% of stock buybacks and completely rebuild our entire society from the ground up
There isn't plenty of money in fact. Your 5% tax would get you $30 to $50 billion depending on the year. You're not rebuilding much with that. You can build a few miles of NYC subway with that, and maybe a quarter of a high-speed rail line in part of California.
You need to fill in the trillion dollar budget deficit first of all, which overwhelmingly consists of rapidly expanding entitlement spending deficits where we're not bringing in enough tax revenue. There is only one way to mathematically accomplish that in reality: raise income taxes dramatically on the top 1/3.
That only gets you back to break-even. It doesn't pay down the federal debt ($600b per year in interest) and it doesn't provide any expansion for spending programs new or old.
The US is already spending as much on its total government system (Federal + State + Local) as what advanced welfare states in Western Europe spend in total on their government systems. And we're getting a horrible return on that money almost across the board. The first thing we should do is improve the effectiveness of the money we're already spending while we attempt to bring down the deficits. For what we're already spending, the whole of the US should look like Denmark or Sweden. Why doesn't it?
Everyone likes to say one thing first in response: military spending. Ok, slash $150 billion off of that to bring it down to 2.5-3% of GDP type levels (and freeze it for a decade). Next you need another $850b to $1.2t minimum each year in new tax revenue just to balance the budget this decade. It will require immense income taxation increases. There are no other sources that can come close to filling that epic hole.