So you'd prefer developing for some obscure OS with a tiny number of users?
Using that logic they should target only e.g. MINIX (mentioned in my previous post, google for details) as it is the original open source OS. Sure you might only have 10 users, but that does not matter?
If you are indeed a professional software developer, you do need a reasonable number paying users to provide you with a salary, or you wont be able to pay your rent and food.
No, I don't. But I prefer to know the specific requirements of the application's target users and design an artifact matching their needs. For me that would be a successful design. It has nothing to do with the size of the user group, except that his goal is harder to reach if the group size increases.
You mentioned the paying and salary aspect. Sure you need to earn money. But in an open-source context there are no paying users. At least not for the code. Maybe you get donations if you are lucky. If you don't have funding than this type of development is for your spare time unless you work in an academic context. To me, there are no other reason than money or reputation to argument the desire for a large user group. Even not for quality (think of openssl)!
And no, KiCAD should not shoot for MINIX. MINIX never was designed as an operating system for the average user. It was specifically designed as an educational system for Tannenbaum's lectures on operating system development. I'm glad you brought this example as it clearly shows how successful a design can be with the right target users in mind. Thousands of students read his book about MINIX (including myself) and one of them created Linux in the end. What a great success! Had the aim of the project been to maximise the user group size, Tannenbaum might have been better off developing a DOS clone instead...
However KiCAD does the other way round! They already have found their niche and have developed a pretty decent system that could be working on other platforms than Windows equally well for a large group of users. But instead carrying about those platforms they willingly accept to shrink their community by pissing off non-Windows user (Yes, under OSX it is equally bad as on Linux).
As I already said: Building open source software that is only useable on closed source systems is a contradiction to me ... regardless of the size of potential users ...
Anyway, happy coding ...