There's also TCAS, which is transponder based and will negotiate with nearby aircraft also equipped with transponders - so if there's a collision risk, the TCAS

There's also TCAS, which is transponder based and will negotiate with nearby aircraft also equipped with transponders - so if there's a collision risk, the TCAS in one aircraft will be saying "climb, climb" and TCAS in the other aircraft will be saying "descend, descend". Or both TCASes might be saying "traffic, traffic" if someone else is nearby, and not on a collision course, but could end up on a collision course if one or both parties make course or altitude changes. So in this case it's a heads up to look for traffic nearby. This doesn't take away from anything you said; I just wanted to point out that a solution based solely on self-reported positions can be quite effective too.