This is a common occurrence in Linux kernel development. If something is going to be done at all, it should be done right! Kernel developers are not averse to rewriting large amounts of code in the name of perfection.

However, the actual addressable memory may be less than the word size. For example, although a 64-bit architecture might have 64-bit pointers, only 48 bits may be usable and addressable. In addition, the total physical memory may be larger than the word size, thanks to things such as Intel's PAE.