an SD card is just a card with EEPROM chips. There are a limited number of read/write cycles for this type of design before failure, typically a few thousand. The memory storage of your phone can only be of one or two types. First is RAM this uses constant power, and will rapidly wear down batteries. Second is the EEPROM discussed above. The chips will wear out with use. Sorry, but that is just physics.
There are a few other options, but those are either too expensive, or not ready for commercial use yet. The advantage of SD verses internal EEPROM is that it can be replaced, instead of replacing the entire phone. Yes, it is expensive, but $60 is still cheaper than $400 for replacing the phone because it no longer works. That is the real reason that paging is a sub-optimal solution from a design standpoint.