Legacy computers (e.g. minicomputers like DEC & HP and mainframes IBM 360, and larger) are typically computers that simply put, have become 'to expensive' to maintain.  When comparing the cost of buying raw processing power, there's no comparison in:

• CPU seconds,
• Millions of Instructions per Second (MIPS),
• Storage cost per MegaByte,
• Etc.

These costs have all dropped so substantially that most manufacturers and heavy CPU users have re-focused their software on comparatively inexpensive PCs and 'blade' servers (e.g. servers that can hold and operate many processor blades).
   
So, 'legacy' is just an adjective we use for something that is, or is getting closer and closer to being, obsolete.  Our guess is that the term 'legacy computers' will be around for a long time, and before long will come to include the heavy and clumsy-sized Pentium 2, & 3 (Soon, or already Pentium 4?) PCs that were typical new purchases just a few years ago.  We see the term 'legacy' as being much like the term 'artificial intelligence'  in that it's meaning will grow to include more-and-more of our old computers, in the same way that the term 'artificial intelligence' grows to include more-and-more of our expectations for new ones.
 
PS  Where is the end of the technology's progress and the computer revolution?  We think it comes just after the artificially intelligent computers we work with, talk to, and take instructions from, are just a little smarter than us!

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