I know like a decade ago, steam emulators existed that could replace the steam DLLs for offline games (or potentially, but more risky: online games that don't rely on valve's servers).

I've not looked into it since, but honestly I'd be totally unsurprised if they never engaged in a war with the pirates on those tools, and they are no harder these days. My impression has always been that the Steam DRM is more intended to prevent casual copying than to be a real piracy deterrent. (This meshes with games on steam often including more invasive DRM systems on top of the Steam one.)

What about all the piracy that happened because of intrusive DRM? It certainly prevented that kind of piracy.