I'm not looking for a number, but rather for a way to make an estimate taking things like market capitalization and the fact that mining uses ASICs into account. (I assume ASICs are relevant since their usage implies that an attacker can't get very far with general-purpose hardware from a botnet or public cloud.)
My motivation for asking this question is as follows. I assume the current rate at which energy is expended on Bitcoin mining is primarily the result of high rewards. As the 21-million supply limit is approached, mining will most likely become less profitable unless transaction fees increase immensely. At the time I'm writing this question, transaction fees only account for a small fraction of miners' profits, while minting new coins is the primary driver of profits. At the supply limit all minting stops, so transaction fees are bound to increasingly become the major (and eventually only) mining reward. It seems inevitable that mining activity will decrease as a result.
Given that mining activity is likely to decrease significantly in the medium- to long-term, how much is needed for Bitcoin to remain reasonably secure?