ACAS using some other technology than TCAS/ATCBI has been proposed and from a technology point of view appears feasible. Question is as I understand TCAS to get the ultimate in service from it the other aircraft must be TCAS equiped or Mode S xponder equiped. A regular ATCBI transponder is okay but doesn't get the ultimate service. So if some people are automatic dependent surveillance broadcast (ADS-B) plus xponder and others are TCAS the overall benefits to all will be less. Of course UPS/AT is an avionics manufacturer and has it's own point of view on whether more better boxes are the solution. (of course they are!!)
The question of which came first, the procedures or the technology (or the requirement, for that matter) is fundamental. Setting aside the formal requirements identification stuff it seems to me that the procedures and the technology evolve hand in hand. The system we have today is based upon VHF am radio communications and navaids and seems to have come almost as far as it can and it is now time for a quantum leap in technology and procedures. I think everyone could come to consensus on what future procedures and technology should be, but getting there from here is the hard part. And doing it while competing with other investments and opportunity costs adds a further complication.
To reduce the overall risk for everyone the procedures will have to evolve substantially quicker than they have in the past taking their own sweet time. Possibly a way to evolve quicker is an industry/regulator/service provider joint trials activity/virtual organization to use simulators and real crews/controllers/engineers to try things out and evolve procedures faster than would otherwise be the case. It would be much easier to try out what may be a high payoff procedure development in the simulator with no metal or people riding on the outcome than it would be to try it out for the first time in the real world.