If we assume constant number of miners and constant spending on hardware and electricity, difficulty can be modeled d=c*2^(t/18), where t is time in months and c is some arbitrary constant. Plot the log of that to see payoff over time, it's linearly increasing, not exponential.

That's not to say we really want linearly increasing payoffs - one would further modify this function to set how the money supply increases over time. How exactly depends on your economic school. We could even aim to slowly reduce money supply increase similar to Bitcoin.