Tcas will only ever be reliable if all aircraft are forced to be transponding. That is the case flying in the IFR invironment of airways.
The downside out of controlled airspace is that Tcas can give a false sense of security and hence in a certain respect make a collision more likely.
So its really an all or nothing game ie make all aircraft use working transponders ie gliders, microlights light aircraft even balloons before they can legally fly.
Most of us who fly in IMC conditions out of controlled airspace are at risk especially as one poster said when making a cloud break blind to VMC and not knowing what aircraft are circling or flying below the clouds.
Maybe we should consider the possibility of a complete network of radar coverage to be allowed to fly in IMC offering RAS or RIS services but even that is not foolproof especially low level.
The only "almost foolproof system is a combination of both ie TCAS and radar from takeoff to touchdown or controlled airspace.
Pace