I think the other lesson to learn is that “everything is a …” abstractions don’t work as well in practice as in theory.
All abstractions leak. The more surface area an abstraction covers, the more leaky it is likely to be.
All abstractions leak. The more surface area an abstraction covers, the more leaky it is likely to be.
But if you've ever worked with it then you will be longing for exactly those features forever and wondering why we did not manage to take more of its deep insights forward.
A lot of really good ideas have been slaughtered on the altars of commerce and perceived performance improvements. As a result we are still developing software roughly the same way that we did 50 years ago, a model that wasn't ideal but that was good enough.
Making a simpler API is a great idea, and from you comment I guess they mostly succeeded, but they did not have to go down the everything-is-a-file path to do that.
What role this played in lack of adoption depends on what alternate course of action you compare it to. A non-file-based rewrite of the API would likely have had approximately the same result, in any case a good compatibility layer might have made a big difference.
Everything is a file is even slower than micro-kernels IPC.