The problem is, at a certain point, catering to low-end machines will impact everyone else and reduce the overall quality. This wouldn't be an issue for a non-networked game, or maybe a game that lets someone host on their machine, but using Unity's online servers brings a few headaches. 1) low-end machines that can't get 60 fps (even on low settings) can cause desync and rubberbanding, problems with animation sync, var and event syncs (health, shooting a weapon, etc). You also get potential issues with dev work for networked objects as they have to be redone to accomodate for low end machines that struggle to pull 60fps at med settings or lower.
The specs I wrote (core i3/4GB ram/GTX 700) were a rough estimate for getting 60 fps @ med - high settings (again, I think it wouldnt be worth playing on low graphic settings. no shadows, low res textures, etc). Not that it's set in stone, but I would say it's something like this:
Test Machine/Recommended (60fps @ Max Settings @ 1920 x 1080)
Core i5 7300HQ
8GB RAM
GTX 1080
Minimum (60fps @ Med/High Settings @ 1920 x 1080)
Core i3 7th Gen
4GB Ram
GTX 600 - 700
Playable (60 fps @ Low/Med settings @ 1280 x 720)
GTX 500 (so any GTX from 2010 onwards)
Intel Pentium (2011 onwards)
2 GB RAM