Both aircraft in the DC collision had transponders on and were tracked by web sites that gather such data in real time and both were in radar controlled space overseen by a controller in communication with both aircraft, so anyone who wanted could see them come dangerously close. Lastly, Flight 5342 received a TCAS alert 19 seconds before impact, which wasn't acted on. Both crews likely suffered expectation bias, making assumptions about the location of the other craft that weren't valid, so didn't actually verify with tools already at their disposal. There is no reason to believe an alert from ADS-B In in both aircraft would have affected the expectation bias.
There was a fatal midair between between a C-152 and a C-340 at Watsonville airport (WVI) on August 18, 2022 in which the C-152 was using ADS-B In and reported he could "see" the C-340 coming from behind him and the C-340 reported looking for the C-152 on his left (the C-152 had already reported on final though.) They still collided despite attempts by the C-152 to maneuver away (not that much happens very fast in a C-152.)
I think anyone who has used ADS-B In knows its worth and limitations (ironically less useful in a busy pattern IMHO) and requiring it is typical government paternalism. Sure, require military to use ADS-B Out unless absolutely needed for operational security. The guard military helicopters based at Rapid City always seem to have theirs on. The bombers at Ellsworth never seem to, not sure they are even equipped, because whenever I hear them overhead they never appear on any ADS-B tracking web sites.
