QE class won't go for 50 years but you can't procure ships on the basis that they will be knackered after 25....So you programme them to last for 50 years knowing full well that they won't reach anywhere near that mark. Bear in mind however that the stipulation was that it should remain in service for 50 years....Not remain in British service for 50 years.... In 2050 when they are 35 years old we will sell them both to India.
However, there are plenty of examples of equipment still going after 30 years... Lynx Helicopters entered service in 1977, INS Viraat (HMS Hermes) has just come out of its 11th refit and is now 47 years old, HMS Liverpool is 32 as is HMS Manchester. Sea King HAS 1 first flew in 1969 making it 41.