These old transponders will decay at an increasing rate. Some of the internal parts are perishable with time and others from use. eg. Electrolytic capacitors and the output tube.
Similar for radios, though many of the tube models have already gone Tango Uniform.
Yes, there will be an ever increasing supply of ancient radios & transponders for low prices, though this pool will evaporate with age. I imagine an increasing amount of pilots will opt for the new and highly reliable.
We will probably get a few posts about "Back in the day these old avionics were overbuilt and highly reliable". True, and so were American cars with 3x thicker steel. IMO, were much closer to the absolute end life of these old units than some desire.