No tax authority, third party, investigator can ever know who owns these freshly minted BTC with any amount of effort or pressuring exchanges to give out their wallet addresses.
Oh really?
To actually have a credible chance of mining a solo block you will need at least 100 miners, that would be around 400kw of power and power consumption per month. Getting over the fact of importing gear worth 1 million and get it pass customers, setting a farm, consuming 300 times the average us household electricity or 1000 those of Germany, do you think you will get undetected?
Miners leave far more traces than a guy giving 10 bucks to a homeless guy to buy some coins at an BATM, besides, if you're mining on a pool, the moment the pool sends those coins your way they are already used or dirty, so, at what could you use them different than coins purchased for an exchange?