So ... how does this work after bitcoin has been successful for several decades, and we have trillions of past transactions, but only hundreds of millions of active users providing meaningful compute power for mining?

In other words, does the cost of mining increase over time with a higher exponent than the number of computationally useful mining nodes?

I am quite willing to agree that bitcoin as currently conceived is not well suited for micro-payments, without some additional supporting infrastructure, outside of the core bitcoin mechanism.

However, it might be possible, once such a full fledged analysis of these factors had been done that this would illuminate the way to core algorithmic improvements in this or other useful regards.