Monero is difficult to track. Bitcoin and ethereum and most of the rest are not. The problem is that fewer and fewer exchanges allow you to buy monero. It will be around as it has become the preferred currency of the dark markets and it can't really be shut down because it's decentralized, but it would be difficult to build a shadow economy on it, or any crypto for that matter. I had enough trouble teaching my mom how to sync her phone to her car's bluetooth. Trying to get millions of tech noobs to properly and securely obtain and use privacy coins for their commerce without screwing something up and ending up in jail seems pretty unrealistic. Plus if it ever got big enough, the federal government would go completely nuts trying to come after people for tax evasion, etc. The psyops would go through the roof. Most people would fold. They'd have to really be dedicated to keep it up in the face of such opposition.

The shadow economy idea sounds good and is technically feasible but extremely unlikely to grow past a small cadre of devoted ancaps and the like.