ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. (Basically, if a transponder used to be optional before, ADS-B out is also optional.) Mode-C (altitude encoded transponder replies) are also not required, but the equipage rate is very high at this point, which is less the case for ADS-B among GA aircraft.
> ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL.
Sure, but so what? The lack of universality is not an impediment to implementing a new TCAS system that uses ADS-B data when it's available. (And BTW, the places where ADS-B is not required are generally places where traffic is sparse and so the risk of a collision is pretty low to begin with.)
If the expense and lead time for the redesign now that more reliable and precise position (and first derivative thereof) are available from GPS via ADS-B are to be considered, it's fair to ask "well, what are the downsides as compared to using altitude for separation as today?" and one of them is "a higher percentage of aircraft are transmitting mode-C than will be transmitting ADS-B out".
Given that the clear priority is now to follow a TCAS RA (resolution advisory) over an ATC instruction, I think that the current TCAS approach is good enough to cause aircraft to miss each other and that a GPS-based redesign is unlikely.
https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/whe...