Bitcoin mixers are still useful, as exchanges can still flag transactions and can still monitor things like this, even when using XMR. For example, if you change $1,000 of BTC to XMR, and then a bit later you change $1,000 XMR to BTC, those transactions could be linked, especially if you did this repeatedly. Also, most reputable exchanges are KYC in order to comply with the law. That's not to say all non-KYC exchanges are bad, just that mainstream/KYC law-abiding ones are more likely to be trustworthy.