Disclaimer: To the best of my knowledge, the 4004 was manufactured from 1971 to 1986, an astonishing 15 year product lifespan. This claim is based on 1986 being the last date code I have seen on the "new old stock" electronics surplus market. This seems like pretty solid evidence to me, but it is entirely possible that my dates are off a bit. I welcome any documentation to the contrary.
Why did the 4004 family last so long?
One can only speculate why it was manufactured for so many years, but unlike Intel's later microprocessors that required a printed circuit board full of chips to implement a complete computer, the 4004 family offered an elegant, scalable architecture that, in most cases, needed no "glue logic" (e.g. 7400 series decoders). The simplest of 4004-based designs can be build with just two chips, a 4004 CPU and a 4001 ROM. The 4001, besides providing 256 bytes of read-only program or data storage, has four I/O pins that can be individually factory-programmed to be either inputs or outputs. Add another 4001, and you get a total of eight I/O pins, etc. Similarly, each 4002 RAM provides four output pins in addition to its 40 bytes of read/write data storage. The 4004 can address up to 4k bytes of ROM and 640 bytes of RAM, small by modern-day standards but remarkable in its day.