For example, even with non-reused addresses, compliant participants may report their addresses to a risk-scoring service. If you go outside that "system", boom your coins risk score goes way up.
This is not sustainable in the long term.
All that's required is that users put one non-reporting entity between every transaction between reporting entities. A non-reporting entity could be anyone or any business.
Coinbase
et al. may attempt to keep their users inside a walled garden that is only allowed to transact with other people in the same walled garden, but they won't get away with it in the long term.