Even with a pessimistic estimate about the rate of progress, fully general AI will be here loooong before population decline decimates the economy, as will good robotics and all the stuff needed to basically put resource gathering and production on autopilot. At that point "on welfare" would easily be the equivalent of living like a millionaire today.

The bigger questions are whether global warming or WW3 will fuck us all before we get there, not to mention the question of whether fully general AI will be its own mega disaster.