Fossil fueled GA is a very non environmentally friendly pastime relative to its benefit to society on the whole.
Why not add all forms of motor sport to that? I'm sure that there's a far greater number of people involved in all kinds of fossil fuelled motor sport than there are in GA.
What's the difference between caning a car for a couple of hours on a track day and flying a light aircraft? Similar cost, similar fuel consumption.
I don't see motor sports declining anything like the decline in GA, certainly not in the UK at any rate - quite the reverse.