At first this seems like a worrisome topic with no solution.

Expenses paid to secure the network, if unnecessary and unchecked, could raise bitcoin fees enough to let other currency designs overtake it.

However:

What happens long-term? Does the incentive for ever more mining gear wind up consuming all available electricity and all possible ASIC/quantum computing resources ?

Optimistically I think not.  Again, long-term:

> New clean energy sources (LENR, space-based solar, fusion, etc.) are likely to come on line, drastically cutting the cost of energy and the pollution load.

> Mining (if not redesigned) becomes ever more concentrated in several pools.

> Bitcoin price should eventually start to plateau, while mining rewards decrease. This raises the barrier to entry for new miners. No longer would mined coins increase in value so drastically: it may become permanently more cost effective to purchase coins than equipment.

> At that point, the remaining pools lose their incentive to keep growing. Art Forz (of GPU & FPRG mining fame) once commented (on IRC) that he discovered trying to control more than about 1/3 of the network leads to a situation in which one is essentially competing against oneself (diminishing returns are realized for additional investment).