Fair play, regarding the tooling being there then, I had the impression it wasn't even possible currently. I guess I'd now wonder how ubiquitous its usage is.
My concern with your second part is that law enforcement would not be able to quickly deal with the issue and in the case of an abandoned relay, could take a fair few days or weeks before any action is taken. The problems with such illegal content is that in many places even unwittingly having it in your browser cache would put you massively at risk—it needs to be removed and the user prevented from continuing as immediately as possible, anything else puts the people using the network at risk. If such a risk exists, it's going to put most people off (and entirely understandably). I know I avoided browsing lemmy for a fair while when the problem here was still being figured out, and I thankfully never saw anything, but I'm still weary of browsing on my lunch break at work for example.
Also FWIW, I think Google does scan emails and drive for this stuff, I think all US based social networks have an obligation to do so also, IIRC, but I might not be 100% correct on that.