While hardware is expected to remain useful on timescales exceeding
ten years (PDP 11s and IBM 470s are still functional machines today), software
evolves on timescales less than two or three years. Long before
the machines stop running, companies stop providing support for their
operating systems, development tools and environments, networking upgrades
and other necessary software. In the case of networking tools related
to the Internet (like World Wide Web), change is currently taking place
on scales nearer to months. In addition, the size and complexity of the
programming environments continues to increase as developers build for
larger and larger memory, discounting the continued use of small memory
machines.