Well, I'll take the bait, I suppose.....
As has been pointed out by other posters, GA still relies on magnetic headings. (It is not true that a GPS will give you a true heading. It may give you a true track, but not a true heading, and the two can be very different. Besides which, many people fly without a GPS.)
It would certainly be possible for air transport to operate on true headings, and GA to work on magentic headings. But could you imagine the headache that would cause ATC? Isn't it far easier to have everyone working off the same reference?
It's very easy to get the equipment on an airliner to convert true headings to magentic. It's much harder to get the equipment on a typical light aircraft to convert magnetic headings to true. Therefore, the lowest common demoninator is the magnetic heading, and that's why we all use it.
FFF
--------------