The monthly fee is based on the fact that non-one or at least very few people will ever play for every minute the server is up of the month subscribed. The amount charged would be too low to be a reasonable charge in this instance.
The charge you pay is a reasonable balance of the average hours the mmo owner expects you to play for. If you start pushing to get your monies worth the MMO owner will increase your monthly sub to be more reasonable, although in your eyes you will no doubt consider it unreasonable.
See if you can find the sub costs for APB. They were doing both giving the player the option of which method they wanted. At the time I calculated that I personally would be cheaper going for the monthly sub option.
As for the lifetime subscription, no company will ever admit why, but why do you think companies like this give a "lifetime" sub option for just over a year's subscription?
WoW is way too popular with the current subscription model for Blizzard to consider the alternatives. If the alternatives have the potential to make Blizzard more money then it is highly likely only then will they consider adding those options.