I'm saying that "You don't need to hit a huge number of service providers because the USA market is highly concentrated" is not a good argument because the act of taxing the concentrated incumbents will cause changes.

Also note that the micro-service providers might just be white-label resellers of the big providers.

Which supports my general point that services are hard to tax because unlike physical goods you can't force them to flow through a few physical choke-points. The fact that economies of scale and other factors cause them to naturally collect in a few concentrated firms in the absence of taxes does not mean that you can just tax those concentrated firms because they will figure out ways to not get taxed!