To add, I believe the biggest factor in the death of the Lisp machine market wasn’t the software, but market shifts in the 1980s. For one, Lisp had a strong association with symbolic artificial intelligence, and during the AI boom of the 1980s, Lisp machine vendors such as Symbolics and Xerox were successful selling Lisp machines to companies and institutions that did AI. The Reagan-era military was a major customer of Symbolics Lisp machines, if I recall correctly. However, this AI boom was followed by a long AI winter that started in the late 1980s and didn’t end until the machine learning/big data boom of the 2000s. By then Lisp was no longer the language of choice for AI practitioners; it’s now largely Python (and sometimes R) backed by numeric computing code written in C, C++, and even Fortran.

In addition to the AI winter, Lisp machines were facing performance and cost challenges from RISC workstations. Suddenly not only did Lisp programs run faster on Unix workstations, but they were cheaper, too. Due to the combination of the AI winter and competition from the Unix RISC workstation market, many Lisp machine vendors exited the market, pivoted, or shut down in the late 1980s and early 1990s. Symbolics Open Genera was part of this pivot; it is essentially a Symbolics Lisp Machine VM originally running on top of a DEC Alpha processor running a proprietary Unix from DEC.

It’s just unfortunate that Symbolics and Xerox didn’t make Open Genera and Interlisp-D (which was similarly ported) more available, which could have been a strong contender to Java in the 1990s (the Common Lisp Object System blows the socks off C++ and Java) and to Python and Ruby in the 2000s, and could’ve introduced many of the benefits of having such a flexible development environment to a much wider range of developers at an earlier point in history…who knows what tooling we’d have today if we picked up where Lisp machines left off in the 1990s. The closest we’ve gotten to these Lisp environments are GNU Emacs, Racket, Squeak/Pharo, and Apple’s work in the 1990s on Dylan and SK8 (this would’ve been particularly revolutionary if it weren’t for Apple’s precarious state in the mid-1990s).

But, alas, the industry took a different direction…but at least Interlisp-D is finally open source, and thanks to Hacker News and other sites more people like me who never used a Lisp machine (I was born in 1989) get to learn about them.