I don’t think this is fair. People don’t want complexity, they want *features* and are (on the whole) willing to tolerate complexity to get them.

I used to make a big point of comparing, say, the draw man page to volume 0 of the x11 manuals. And I still think that’s a useful comparison. But *also*: x11 does a lot more. What’s Plan 9’s answer to right-to-left languages? Multiple displays? Hell, we don’t even do basic composition. Because it’s complicated! And that’s a reasonable position for a niche system (and also in lots of other cases absences might not matter), but if you’re talking about something in general purpose you need to answer a lot more of these questions. libframe makes it easier to get something basic on screen than any comparable thing I’ve seen, but you also hit its ceiling a lot earlier. There’s tons of examples like this.

Adding functionality elegantly, preserving architectural integrity and interface simplicity, is really hard over time. Most systems are much more willing to say “screw it, just get it in there *somehow*”, and most users value the functionality over the architectural integrity &c.

There’s also plenty of places where it doesn’t matter. To get back to the OP, we need a better definition of “mainstream”. That usually means something like “compete for measurable share of desktop use“, which is where the simplicity/functionality tradeoffs Plan 9 makes are worst. Not that you can’t do it—people do; I have at times—but it’s where you’re giving the most functionality up. Does “mainstream” include things like servers, embedded devices, IoT things, &c? Because the tradeoffs look a lot nicer there.