Don't forget that 64bit removes the limit on the size of the proc cache. Under 32bit SQL proc cache was limited to 2GB so it would never munch its way throught he buffer cache.

Some businesses migrate on a cost vs benefit principle. I have software that works better than it needs to running on platform 'x', why would I pay for an upgrade for no tangible benefit?

The fact that it runs on old hardware that might break or causes headaches for us in IT is seen as irrelevant.