I think the point you bring up on bitcoin's blockchain security is interesting. Lower rewards mean a lower incentive to mine. And rewards are going to be near zero after some point too.
The monetary incentive from that point on would be fees. Lower or close to absolutely no block rewards also increase the interest miners have in the maintaining of a fee market.
So paradoxically, after that time before 2140 comes, parties that transact BTC will be providing the sole monetary incentive to upkeep the safety of bitcoin's chain. But a conflict here is that the same parties will also want cheap transactions.
Right. I agree. A question that arises from this: is there a monetary amount that we can state as $X USD which would provide a acceptable amount of network security? Such that X amount is purely spent on mining (i.e. including human resources for maintaining rigs, electricity for running rigs and cooling them. Not including costs such as buying new mining hardware, development of liquidation algorithms, etc.).
I assume this is a complicated question, as electricity prices are different in different locations, miners have different efficiencies of converting electricity to computations.