I have to wonder how much tax would be generated. First, let me state that I prefer the more limited view of cryptocurrency broker because I see too much cost expended to collect too little extra revenue with the more expanded view. Those supporting the expanded view may have a different thing in mind . . . end cryptocurrency. I say if you want to end cryptocurrency, you should say so and act more directly.
How much cryptocurrency value is from fiat currency conversion directly and how much is due to the rise in "value" due to speculation? The US is currently treating cryptocurrency as an investment and taxing as such, only when the the conversions between crypto and fiat and vice versa take place. If a relatively small amount of fiat is converted, then a relatively small amount of revenue on that end. If a relatively small amount of crypto is converted to fiat then same. I suspect most of the "value" of crypto is not going to be converted to fiat and that "value" may virtually disappear should a run in crypto cash out occur.
There may not be as much revenue to be had as the politicians think. Even worse, US tax law also allows for consideration of investment losses to partially reduce tax liability. Those details may or may not apply to crypto investment, but they have to think it through.
As per tax evasion, defining brokers as primarily exchanges would help on that aspect.