Do the IRS have the resources available to go after these 50-100 million users?
Maybe. Maybe not. They certainly have the resources to send mass threatening letters to everyone, and one would imagine their database is sophisticated enough that they could simply pull the details for everyone that Coinbase/Kraken/whoever tell them about and see if they have reported cryptocurrency gains, or even something as simple as marking "Yes" to the cryptocurrency question which now appears on everyone's Form 1040.
If the users had used these exchanges for just one or two trades, then what is the possibility that the IRS will go after them?
I suspect they aren't examining actual reported gains and losses against the data the exchanges are providing them except for people with high trading volumes, because that takes a lot of manual work and is expensive and time consuming. However, if you have a fully verified and KYCed exchange account, but you have marked "No" to the question mentioned above, or haven't filed a Schedule D, or haven't filed a Form 8949, then that can be flagged up with a simple database search and is a pretty sure fire sign that you aren't giving the IRS what they want.
At the end of the day, none of us can be sure. But if you have a fully KYCed account, then the IRS will get your details eventually. Whether or not they end up investigating you further is anybody's guess. If you are concerned about this, then you should take the advice of a tax professional.
And everybody should stop handing out their KYC like confetti to anyone who asks for it.