I don't think decentralized indexing is a solvable problem. There's too much crap, and too little in the way of useful signal to tell it apart.

I've been using decentralized indexing for more than a year and I haven't needed to go back to ad- and malware-ridden search sites. I find the number of seeders a good enough indicator of quality, but of course that only works for popular content.

EDIT: thanks for formatting, I always forget this double linefeed rule