If you bought KYC, they know how much you bought. Mixing it doesn't take that fact away.

Imagine the conversation:

Gov: You bought N bitcoin on this date at this price, please pay unrealized gains taxes on it.

(mixing seems irrelevant here)

The Bitcoiner: Um, but I don't own the coins anymore.

Gov: Prove it. Show us your transaction trail and where it was sold, and the CGT you paid at sale time.

The Bitcoiner: Actually, I lost the wallet with the coins in a boating accident.

I'm not sure how they will try to prove whether someone does in fact own it or not.

With AI (or smart pattern analysis algos) it's likely they can identify correlations between KYC fiat income (eg. bank debits) and transactions downstream (post-mixing) of your KYC addresses they know.  Off-ramps will be heavily monitored to find these I'm sure. 
So cash would be the only safe transaction, if they aren't watching with drones / street cameras and AI at that time.

Man I'm super black-pilling now. Haven't actually thought about this before..