Not so mainframe hardware is under constant development and newer faster shinier machines are released every thre or four years.
Shiney enough that most Banks replace thier production kit every five years or so.
An obsessive commitment to backward compatibility means in most case it just a case of wheeling out the old machine and plugging in the new one.
Software is a different matter the above mentioned obsession with backward compatibility means you can happily run software written in the seventies.
The worst offenders in the obsolete hardware stakes are Windows servers, systems stuck on obsolete operating systems and out of support databases because of the expense and risk involved in each upgrade.
Although my personal best was in 1999 I saw a pdp-11 running a production mail server. It was due to be replaced because no Y2K fix was available. So it went to a museum after 25 years service.