Imagine you're part of a development team where the tech lead in every thread says "I am the tech lead because I'm the smartest".
It's made me contribute less to OpenBSD than I otherwise would have (luckily other people are more welcoming, and Theo isn't a bottleneck on all things), and it's not just me. Other potentially good contributors have stayed away. Now, of course, other bad contributors have stayed away too.
The other bad aspect to arrogance is that it misses out on research the rest of the world has done, because you think nobody else can think. OpenBSD got W^X years and years after Linux (though OpenBSD got it by default first), because (in their own words) they don't look at what Linux is doing. OpenBSD missed out on W^X for years, and it took one more release for x86 to get it, because they said it couldn't be done (even though it worked just fine on Linux).
Looks like it was the same with the intel branch predict bugs. They say they did huge amount of research over weeks or months, and then just ended up with the kernel memory maps containing... exactly what Linux chose. Why did they do this from scratch?
I wouldn't say I'm bitter, but resigned to just accept that OpenBSD's way of doing things misses out on exactly what they want to achieve because of this attitude.