Maintenance and technical refresh of complex 100% availability systems is a very expensive and complex business and most organisations with such systems do not handle that well at all.
It requires the correct people to be retained and knowledge to be properly documented.
Conversely, for many such systems, the actual job the system has to do does not vary greatly over many years.
In addition, the probability of a replacement system being introduced on more recent technology and more up to date development methods/languages etc without a loss of availability at least in the early stages is not high.
Hence, for most such organisations upgrades are very rare unless the capacity is reaching some kind of limit.
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