"Good software" is not some mystical status of code perfection, it's doing the things its users want it to do. So, yes, popularity equals good software.
Software, as any engineering effort, is measured by its utility and use. It's not like art where popular is not necessarily good. Unless you are some kind of dreadful "code poet", whose programs nobody uses.
But notice how I didn't just say that Apple's software is good because it's popular. For example, I challenged anyone to mention a "substantially better mobile OS than iOS".
>Regarding mobile OS, we all know this now depends on the number of "apps" available.
No, even without apps, the basic OS is just as important. iPhone, at it's introduction, had by far the best OS of every other smartphone out there, even without third party apps at all.
>Many may have made a superior OS, but adoption has been too low because of the lack of apps.
Really? Disregarding apps at all from the comparison, do you care to mention one "superior mobile OS", let alone "many"?