Primary, or Mode A only, returns below CAS are assumed by ATC to be OCAS. So far this seems to have worked, through luck and the sky being quite big
I think that it would take only one GA-commercial incident for much more draconian measures to be implemented.
The bigger long-term picture is that the airlines are generally against GA, both in terms of airfields and airspace access. If they got their way, we would have Class A everywhere. This pressure appears in any political scenario where airspace access issues are being considered. It's important to not give airlines, and those that support them, any more ammunition than the several hundred CAS infringements which they have in the UK alone and many more elsewhere (airspace issues are increasingly being planned at the European level).
I think that if US-style mandatory Mode C veils around major airports were made mandatory say 10 years ago, we would not be facing most of the fuss we have now.