This statement is ridiculous. Symbolics is dead for a number of reasons, but Stallman's Emacs isn't one of them. Stallman copied some of the features in Zmacs. Zmacs was one small part of a much larger operating system which, among other things, included industry leading animation software. There were many, many systems (including the hardware architecture itself) that Stallman could not, and did not copy. No company picked an alternative to Lisp Machines simply because it also had something that looked like Zmacs. Your statement is akin to saying that a modern tablet with a web browser makes desktops (which can run much more powerful tools) obsolete.

Symbolics got used to selling hardware for tens of thousands of dollars and could not fully adjust when competitors were eventually able to sell machines for much less. The company also had a couple of bad CEOs, and suffered large financial losses because of a series of bad real estate deals. Efforts to port their software to other platforms were largely unsuccessful.

Lisp Machines were always a niche market because they were expensive, top-of-the-line machines. The target market never really overlapped with Windows. Cheap workstations that could have multiple users eventually became almost as capable as Lisp Machines and thus displaced them.

If you want to credit Stallman with sabotage then mention Elisp, because Elisp is the worst Lisp still in use (Zmacs ran ZetaLisp).

If I remember the story correctly Stallman saw the new features of the Symbolics software, re-coded them and handed them to Lisp Machines Inc.

So I guess your thesis is that by allowing LMI to maintain feature parity, that fragmented the market meaning neither company could survive? (We're ignoring the Xerox and Texas Instruments lisp machines obviously)

But this is ignoring the point that while this was happening the Mac and the PC were already in the market at a tenth of the price of any of the lisp machines and were selling by the bucket load. Essentially the PC revolution was already well underway before Stallman even started his hacking.

Lisp machines failed because they were selling high end limited use hardware running software that was good for a narrow range of highly specialised uses, but far too resource heavy to be economically viable for most users. The VAX for example was a far better machine for nearly every real world task except at running a single niche programming language.

If you really think single programmer stopped a $70,000 single user machine from derailing the PC revolution and kick-starting whole new world of computing heaven then you and I have a very different definition of "cheap" and without cheap computing we wouldn't be 50 years ahead, we'd be 40 years behind.

Lisp machines (much like the Xerox Star series) were high-end workstations that ran a proprietary OS and didn't compete with PCs. One could build a case Unix workstations killed Lisp machines, but to imagine Emacs held back them until PCs stole the market is quite a stretch.