I’ve read (probably on Wikipedia) that the popularity of Unix was to blame by few for the stagnancy in OS research.
To be fair, it’s not a stretch to suspect a company wants its competitors to be dependent on their product. The theory of planned security vulnerabilities sounds off the plot however.
As elaborated in another comment, I am not a fan of UNIX having won (anymore), but I find this "conspiracy" ridiculous. It's obvious to me that UNIX was seen as a good idea by many people, and that it did make a lot of sense compared to many alternatives, especially considering the constraints of computers and computing environments at the time.