They're not "allowed" to expire, they have an inherent time limit. Of course, smart patent holders make "improvements" and get new patents before that happens.

On the printer market: maybe you mean the laser printer? The 9700 came out in 1977, and the other laser printers a few years after. PARC was founded in 1970.

There was also an antitrust settlement that Xerox made, which forced them to license the patents to competitors before they expired.