The Z80 microprocessor was designed in 1976. 43 years later, its CMOS version is still being produced and can be purchased new in SMT packages. Intel MCS-8051 was designed in 1980, and today you can still find brand new chips that implement the almost-original version. Also, the original version of NE555 timer, the LM741 opamp, the LM317 voltage regulators were still being produced in large quantity and used until recently, despite the availability of their better CMOS counterparts.
One can argue that their continued proliferation in the 21st century was due to the large number of legacy designs and legacy systems, that's true. But, yet, few still use 8080, 8085, or some forgotten chips from the same era.
So apparently there's another crucial reason for the success of these long-life chip: they were made to do one thing, and they did it well, and still are still doing well in low-lost devices.
One can argue that their continued proliferation in the 21st century was due to the large number of legacy designs and legacy systems, that's true. But, yet, few still use 8080, 8085, or some forgotten chips from the same era.
So apparently there's another crucial reason for the success of these long-life chip: they were made to do one thing, and they did it well, and still are still doing well in low-lost devices.