...Absent efficiency increases, difficulty should increase as additional miners are manufactured by mining manufacturers. In general, manufactures will create the most efficient miners, while as difficulty increases the least efficient miners will be taken offline as they become unprofitable to operate (the latter may not always be the case as operating costs vary throughout the world).
The two curves I see are, in order of magnitude of importance:
1) Moore's Law
2) Industrial Learning Curve
Moore's Law capital cost decrease per hash is proportional to inverse exponential with time.
The ILC capital cost decrease per hash is proportional to inverse log units manufactured.
There is good reason to believe (knock on wood) that Moore's Law is coming to an end -- and not just because speed of light constrains the radius of control in CPU clock speeds (as has been the case for almost a decade now). Hash ASICs are immune to the radius of control problem but not the absolute limit on feature dimensions as vertical features are now the last resort in the 7nm and below fabs.
Recent increases in advancement in technology have resulted in the number of transistors per circuit doubling in more than two years. I think further increases in the advancement of technology will continue to slow.
As new generations of miners become less frequent, the ILC will allow the latest generation of miners to be produced more efficiently (over time), but this does not mean that miners will be sold by manufacturers for less. If the market will bear a high price when the cost to manufacture declines, the manufacturer will continue selling at a high price. The ILC plus the decline of Moore's Law means that
producing miners will become more profitable in the future, not that
buying miners will be more profitable.
The above suggests that the rate at which difficulty increases will decline moving forward. This does not answer the question of if it makes more sense to buy bitcoin or buy bitcoin miners because it does not predict the rate at which the price of bitcoin will increase.
