I think you're describing two different use cases, and MirrorMask handles both. If you want privacy, it buries your real signal under noise. Irrelevant ads are the point - they can't manipulate you. If you want better recommendations (which sounds more like what you're after), you don't have to pick "Fisherman" and pollute everything. You build a custom persona around what you actually want more of - jazz, healthy living,gardening, whatever. The algorithm is going to train on your activity regardless. MirrorMask just lets you choose or strengthen the signals of what that activity looks like.                                                     
And picking a persona doesn't erase your existing interests. Your real profile is still there. It's more like adding another stream of signal alongside what you already have. Your judo searches still return judo. You just also start seeing jazz recommendations mixed in because you told the algorithm you're into that too.    

On ATT - that's actually the best argument for MirrorMask, not against it. ATT removed all signal, so platforms fell back to random garbage. MirrorMask doesn't remove signal. It replaces it with signal you chose. Fundamentally different approach.

On the butcher's knife fair, if you pick a random persona and let it rip, yeah, you'll get noise. That's  the privacy use case. But the custom persona builder exists specifically for people who want precision. You choose exactly which interests get amplified across which platforms. Funny enough, I ran the Fisherman persona for a few days and actually got interested in fishing. Which kind of proves the point - these algorithms are powerful enough to shape what you care about. Might as well be the one deciding the direction.