i think it should be pointed out that Microsoft is not alone in this approach. Apple has made it almost impossible to open old documents saved in AppleWorks of ClarisWorks in Pages...
This is a symptom of the approach used for SDLC, Software Development Life Cycle.
When old software is upgraded or new stuff developed, it is done on the latest and greatest OS and registry structures. Few vendors strive to guarantee that nothing will become obsolete.
I won’t say that Big Blue is an exception, but I will say that Big Blue is way, way, way better than most.