Great question - the fundamental entry point of the air traffic system is this VHF control audio. Everything is downstream of that. Trajectory planning misses the intention signal on the frontend.
The example in the original post is actually a good case study for why trajectory planning alone breaks down. By the time the aircraft are on a predictable collision course with each other, you've lost 10+ seconds of potential remediation time that you would've had if you detected the error when it was spoken. Those 10 seconds really matter in our airspace.
One major issue is that radar doesn’t work very well at low altitudes. The lower you aim the beam, the more ground clutter you pick up (trees, buildings, clouds, etc).
Yes, this is a challenging issue in ATC. Also, some safety systems are turned off or diminished at critical phases of flight (near the ground) because of the noise problem (TCAS, for example).
ADS-B helps with this as it's self-reporting. And systems like ASDE-X are useful to track objects once they hit the ground. But low-altitude deconfliction is a big problem.
My understanding is that TCAS is disabled not due to radar limitations, but because it isn't altitude-aware and will happily generate an avoidance command that results in CFIT.
It's been a long time since I worked in flight simulation (full flight simulators and simpler pilot training devices, including simulating TCAS), but I believe at that time TCAS would be switched to a mode in which it only alerts of "Traffic" instead of providing avoidance instructions precisely _when_ entering busier airspace -- e.g. airport proximity. In that environment it was undesirable for TCAS to be giving instructions. That seems like the environment in which Enhanced Radar's (future) product(s) could be of most interest.
(By the way, I believe EGPWS would take priority over TCAS anyway.)
The systems I've flown have definitely been altitude aware. They won't alert if the plane is sufficiently deconflicted above or below.
One problem for sure is that when you're close to the ground you have to be careful about buildings, cell towers, etc. Terrain is one thing, but when you're a few hundred feet AGL, you could quickly be in the way of tall structures if a TCAS alert goes off.
