Personally I think that although getting a job in FT is as hard as it's ever been, the field will remain busy for many years.
But, I think that the nature of the job will change a lot. I suspect that the fast jet military test pilot is likely to find himself becoming endangered eventually, whilst most rotary and large aircraft flight testing is going to become much more systems assessment (and thus FTE) oriented, changing the role of the TP (although far from eliminating it for as long as pilots operate aeroplanes - cockpit design and handling qualities still matter, even if they keep shrinking in importance compared to the ever more complex systems on a modern flying machine).
At the same time, surely the role of flight testing UAVs is going to get bigger and bigger, albeit probably requiring a lot of new skills and techniques from FTEs that most of us don't yet have and presumably maintaining some TP involvement, just because of the unique skills in planning and assessment that a TP has.
So yes, I think that there's a 30+ career in FT still to be had - but you'd better understand avionics and UAVs!
And I doubt that the situtation in GA, where the TP is often required to be multitalented and is often an Engineer as well, probably won't change for the foreseeable. There are very few full time flight testers in GA anyway, nor ever have been.
G