It occurs to me that we have to seperates issues here - GA vs GA and GA vs Airlines.
As WWW says, airliners are being forced to divert to avoid a non-existent threat from GA aircraft thousands of feet below them . Fitting TCAS in this case will be of no benefit to the GA aircraft but of real benefit to the airlines. In this case, it would be nice if the end user was to pay. If a commericial case could be made for the airlines saving money by fitting the entire GA fleet with TCAS, then it might happen.
In the GA vs GA case, it would require us to stump up for the avionics and it would also require a massive expansion in ATC services just to keep an eye on GA fliers. That will just not happen.