David Schwartz wrote:
That's the key difference between the availability of 64-bit processors
and the introduction of 32-bit and 16-bit processors. When 32-bit processors
and 16-bit processors were first available, there was already a huge base of
software that could greatly benefit from the additional capability. However,
there is very little currently existing software that can significantly
benefit from 64-bits processors. Few applications actually need to deal with
numbers larger than a billion, whereas many applications need to deal with
numbers larger than a hundred thousand.

In the case of x86 64-bits, the real gain is to be had from the
additional registers, and the onboard memory controller (in some cases).
Also some unrecompiled 32-bit apps can gain from having additional
address space specifically devoted to them and not shared with the OS,
which now has its own address space well out of the way of these apps.

Yousuf Khan