Thread drift - mandatory TCAS fitment to GA - that would cause some frequency congestion on the 1090 band - even at the Birdsville Races!
If I understand correctly, TCAS sends an interrogation pulse every second, to which every aircraft in range responds to. Thus the number of responses is proportional to the square of the number of aircraft within range of each other - ie 50 aircraft in an area equals 2,500 transmissions per second.
ADSB ES as I understand sends a location transmission every second. The number of responses is equal to the number of aircraft - ie 50 aircraft equals 50 transmissions per second. Admittedly, the transmissions are longer (the ES bit), but they are only transmitted once per second.
A new specification for TCAS could design it to work with ADS-B ES to dramatically reduce the number of responses to TCAS, and as a bonus allow lateral resolution advisories to be offered in conjunction with the existing vertical ones.
If every country waited on other countries to introduce new designs aviation would not exist and we would be still adding up on our fingers. ADSB ES enables a number of solutions to improve ATC, TCAS, CFIT prevention, traffic awareness and SAR efforts, however some international standardisation is required otherwise the wheel will be reinvented numberous times with slight variations. If opportunities are always passed up we will continue to be tied to 1940s technology such as AM VHF for aircraft radio telephony, SSR for aircraft monitoring and US units of measurement.