Just because something is open-source does not always mean there is going to be this magic community of fixers and enhancers. Github is filled with buggy abondoned projects, all of them "open".
There are already enough items to deal with in a security system, I do not think we have a strong need for open software in the sense of open source. We *do* need software and hardware that is reliable, compaitble with a wide variety of products/protocols, and supported by the manufacturer for ongoing fixes and enhancements.
People want reasonable assurance that whatever they deploy today has a strong chance of still being relevant and workable 5 or 10 years from now. I think overall health and stability of the company is as important (more important) than ready access to any source code used to deliver those products. If new standards and technologies come along (like h.265 on the horizon), you want to be able to reasonably assume your vendor(s) of choice will embrace those technologies as they stabilize.
As you point out, I think that Mobotix fails today in most of the basic tests of "am I reassured the choices I make today will still look good 5 years from now?"
There are benefits to a "closed" strategy if you do it right. Microsoft essentially rode that wave for a very long time, and still today manages to pull in a decent chunk of revenue. But other than both company names starting with "M", I don't see a lot of commonalities between Mobotix and Microsoft in their broader strategies.