I don't see this as a Monero problem in particular. There's no point in trading if you don't plan on taking profits at some point, and if you take profit you're going to have to convert the crypto to cash, and it's going to be hard to do this without the involvement of an exchange and a bank. If you do manage to sell tons of crypto to some guy on local bitcoins, you're still going to have a problem when you buy a condo. The IRS will notice and ask you where the money came from. A circular economy where everything is done in crypto will help but won't definitely solve the problem, because the government will know that a title deed changed hands, and if you don't want to involve title deeds changing hands (or legal documentation indicating ownership changes), you risk forfeiting protection of the law.