It is interesting that the ADSB mandate in the US requires that you keep your Mode C transponder.
I've been building GPS receivers for over 20 years (not rocket science as the magic bit is in the module or chip and I just do power supplies and data interfacing).
I do not understand the angst on the part of the authorities about GPS accuracy. Any decent modern GPS chip capable of being set up for aviation dynamics will be as accurate as any TSO'd expensive device. Currently in small quantity these sell for under A$20. RAIM is included but nowadays I haven't seen fewer than 8 sats even on the ground with buildings and trees around and more likely 10 to 12.
The US also allows the use of GPS modules for Experimentals which meet the requirements but aren't TSO'd, the Dynon and Garmin G20x are two examples of these.
There is also the Enigma device out of Melbourne. It will be interesting to see how that gets on with the authorities.