Well look, you called this, "Biggest antisynergy ever"; it isn't. Gunning down fairgrounds with swindler/rabble is perfectly legit and extremely strong, with some setups, stronger than killing provinces to peddlers. Further, there are a lot of other ways to get a curseless setup - for instance watchtower trashing. If the curses mostly get junked as the come in, odds of hitting green on top are much better than purple. Likewise, jack games have few curses, but may allow for province -> peddler swindler. I mean seriously, in a long enough game the curses all revert to coppers anyways.
The point is, curses may not stick around. Heck, you may want to swindle curse -> curse just to end on piles. It isn't the end of the day for the swindler player, and this is far, far, far from the biggest antisynergy ever.
Now is this a bad combo? Sure, in the vast majority of games something else, like straight swindler/BM will be better (or some other rabble engine). But these things don't actively hate each other like some other bad combos (e.g. scrying pool/cache). There are things you can do with them that do work.