There's no such thing as "tainted BTC" or "clean".
Saying that such thing exist is just stupid.
If that will ever exist it will TWO DIFFERENT "COINS" , two different chains and not anymore Bitcoin.
As the recent trials have shown, exchanges and chain analysis companies are grossly incompetent at actually identifying coins so it could be pretty easy to arbitrage without actually giving up anything.
Are KYC coins really expected to be more valuable in such a situation? That's the only way I could see separate markets with separate prices emerging. How much could the difference be? Anything more than few bips would be easy to arbitrage.