In 15 years of working in IT organizations at various companies I've seen hundreds of people run Solaris as their Desktop OS (mostly 1996-1998 at Netscape), various Macintosh System OS's and Irix as their desktop, at times everyone was running Linux as their Desktop OS, and nowadays OS X seems to be popular (obviously there has always been a lot of Windows Desktop Usage)
I have never, once, seen OpenBSD run as a Desktop OS. This despite working at several companies where OpenBSD was actually core to our entire infrastructure.
I'm not saying that nobody does (I expect the OpenBSD developers probably do), but I am strongly suggesting that OpenBSD needs no desktop presence whatsoever to be extraordinarily successful. I've never installed it on the desktop. I've never seen anybody install it on the desktop. Not once in 15 years. There is no rationale reason to run OpenBSD as a desktop, and I'll go further and say I will likely never see X-Windows run on OpenBSD.
And, as for a niche - I consider OpenBSD to be the pre-eminent platform for Firewalling, IPsec, IPv6. Their continual focus on consistency makes the system a joy to use - all of the lessons I learned in 2001/2002 are almost 100% (hell, they might be 100%) applicable to managing an OpenBSD system today.
They may be a niche but it's a pretty darn big one.
With regards to your argument regarding "upstream" support - most of the upstream support required is around elements that are not associated with the desktop, and, in many areas, openBSD has just decided to rewrite the upstream contribuions in their own "OpenBSDsh" way anyways.
I'm confident they will do fine even if upstream support for desktop, and associated applications, no longer considers BSD as a first-rate platform.