I would love to have TCAS. Unfortunately, for the £15k+installation one only sees the planes who have a working transponder. And if they are in Mode A only, one gets lots of reports of mostly-low-down traffic which won't bother anyone flying at say 4000ft. There is an awful lot of traffic in the lowest 2000ft; my guess is that most GA traffic, and probably the majority of XP-less planes (excl. gliders), are down there.
One needs to fly under an RIS and hear numerous reports of "unknown level" contacts, to realise there are many planes (WITH normal electrical systems) which either don't have a working XP, don't have it turned on by accident, or don't have it turned on deliberately.
Without compulsory Mode C, TAS/TCAS will only ever help a bit; it won't reduce the need for a lookout, especially as I suspect that those without a working XP are also a lot more likely to be out of radio contact (by choice).
And gliders are the least likely to carry an XP...
I've had a few close encounters with gliders too lately.
Not a lot one can do....