I do think that if we change ticketing in the future, supplements for certain services e.g. Intercity, or faster/express trains, might be a better way to do things than the current mess we have, with loads of ticket combinations and varying restrictions.
But for now, Gatwick Express is just a name on a Southern train. As such, Southern-only tickets should be usable. It's no longer a TOC, and Southern should have been aware of the implications - as well as the DfT - when changing things.
The Stansted Express is just a normal train (albeit a brand new one, even if they are falling part in places after less than two years!) and they share both GA and Stansted Express branding. The way that GA gets around the 'issue' is by charging a higher fare to go to Stansted Airport than, say, a couple of stations further on (and those who know can split tickets then end short/start early).
In the case of Southern, the problem disappears when the new franchise is created. With no FCC to compete with, there would be no cheaper TOC-specific ticket and so the price will go up, in effect to what the any permitted fare is that's indisputably valid on GatEx.
As I've said before, at the end of the day Southern is trying to keep this problem low-profile. It can currently make more money from the vast majority of ignorant passengers and tourists, and when - what are probably thought of as - smartarses turn up with their Southern-only tickets, just let them off (as always happens). Far better than risk having any action taken that ruins a nice little earner.
I would seriously hope you've not been getting abusive PMs from anyone on here. I think we're all adults and able to discuss and debate without resorting to that.