I certainly don't agree with anyone being grounded because their aircraft is incapable of carrying certain new equipment; I don't personally think the CAA would succeed in making that happen, nor should they try.
On the other hand, as a flyer, I have a moral and legal responsibility to keep safe both my passengers and do my utmost to minimise the risk to those flying in the same airspace as myself. I use an ATC service wherever possible, I also use my TAS as effectively as I can to enhance my lookout scan. Those folk with a transponder fitted get a much better chance of me being aware of their presence in advance (often 10 miles or more) and my avoiding them by a good margin. Those without a transponder don't get that extra protection factor; both myself and the other pilot have to rely on the Mk1 eyeball, with all it's limitations, it's as simple as that.
My eyesight is good enough to see a light aircraft about 4 or five miles away in good average conditions, unfortunately that is about the limit of human visual acuity, Beyond that the "target" is simply too small - our eyes weren't designed to do any more than that. Possibly when we evolved as "hunter gatherers" an animal further away than that was too far away to chase - we'd be cream crackered by the time we got to it.
It's not too uncommon for my TAS and/or ATC tells me an aircraft is very close by but try as hard as I can, I just can't
see the damned thing, often because it's a very small aircraft with insufficient colour or brightness contrast to the background. In these cases, I put on all four landing lights and hope the other pilot sees
me 
. This possibly irritates some pilots, but at least he's seen me if he's irritated
.
Since I began flying TCAS/TAS equipped aircraft nine years ago (28 years prior to that without the benefit), I have become aware just how busy our lower airspace is becoming; far more busy than when I began flying gliders some years ago. Just like our roads are so much busier and more dangerous these days. Back in the early 1970s I also protested against the mandatory wearing of a helmet whilst riding a motorcycle (even though I already chose to wear one myself and knew it made good sense).
I do think that transponders and TCAS / TAS are very good bits of kit to have, much better than seat belts in cars or crash hats on motorcycles because they can, in many cases, help to prevent an accident, rather than try to minimise the after-effects to the individuals involved!
SS, I can't really agree with your example of the unseen sailplane in cloud as an argument against the fitting conspicuity equipment, surely it is an argument FOR fitting it to all aircraft that operate in IMC? Actually, this was the basis of another discussion here some time ago (which went on for ages)! At least one glider pilot voiced the opinion that everyone should fly around on "his" glider frequency whilst in IMC, even to the detriment of obtaining an ATC service elsewhere, just in case he might be gliding in cloud, which he rarely did, on his day off.
