I'd say the biggest reason that Gnutella (and other services like it) is no longer in much use is because for a long, long time now the easiest way to download music for free, without ads, and with virtually zero chance of getting caught, has been via YouTube and downloader clients. For most practical uses, it's good enough.

You wouldn't want to share the resulting downloads (not only is the audio quality slightly degraded, but I imagine it's highly likely there would be audio watermarks), but when everybody can download straight from YouTube anyway with a minimum of hassle, why would you need to share anything other than a video URL?

Of course, a big part of why this is so simple is because of the massive amount of work that the downloader client devs put into working around YouTube's attempts to stop this. I imagine it can be a difficult job.

If YouTube ever win the battle against the downloader clients, I imagine the landscape will change again. Maybe Gnutella will make a comeback.