So will someone now be looking at a scope, looking at registrations, investigation infringements of use ??.
I don't want to be supporting mandatory Mode S but I think it should be said that many places you walk you are already on cameras (thankfully, in my view, if it reduces street crime) and similarly when you drive somewhere.
Already, anybody can look up your name and home address on the CAA website (NOT GOOD) - unless you run the plane under a ltd co. and use your accountant as the registered office.... then HE gets all the mailshots from the CAA and has to pass them on to you.
Almost where-ever you fly you get picked up on somebody's radar; most of the UK is covered somehow, unofficially, and most of this data gets taped. So the powers to be can track you from your takeoff to your landing if they really wanted to. What they can't do is tell your altitude though! But why would broadcasting your altitude bother you, unless you are busting CAS?
Also mandatory Mode C at least would finally make TAS/TCAS worth having - however I am aware that GA mid-airs are very rare so the safety case isn't made.
Technically, the AVERAGE power emitted by a XP is very low and there is no reason why a cheap unit powered by a small battery (weighing about 0.5kg) could not be made. I work in this business and can tell you that the direct manufacturing cost of a typical £3000 avionics item is about £300; the rest is mark-up resulting from the manufacturer being a company (usually a very old one, with lots of middle managers and clue-less salesmen) far too big for the relatively low volumes being made. And no established avionics player will want to bomb the price - why should they?
But someone new could do it. The commercial risk is that Mode S might never become mandatory for VFR outside of CAS and your R&D will have been largely wasted. But a newcomer to the business needs early cash flow - however anyone hanging around GA will immediately realise that CASH is positively the last thing anyone is going to get flowing OUT of the average flying school
And unless the thing is made absolutely mandatory, most schools would avoid spending the money because most can do PPL training VFR outside CAS. No wonder not a lot is visibly happening yet. Anybody who knows the specs could knock up a simple XP in no time at all - it is basically 1970s technology.