Oh, and guess what? Sales figures also impact the business decisions being made by whatever brand meets your camera system needs, too.
I think you're flipping the pancake a bit.
Sales do matter in terms of determining the business decisions of a company. We however are the end users, not the company. To us it can be nice to know why they did this or that, but ultimately it is a very academic interest. Sales figures do not affect our experience once the product is in our hands and do not improve our output or our enjoyment. The first rule of statistics is that it never applies to individual cases.
In the end of the day, I don't really care whether Canon feels the need to update this or that. My money is supposed to be spent on fulfilling my
needs, not Canon's. If their products are good I buy them, otherwise I don't and it doesn't really matter if other people think they're good for them. If I have to choose, I'd rather be a quality minority than a trivial majority, yes.
That said, this point is a particularly moot one when it comes to OEM vs 3rd party offers. The reason why the Canon 50mm sells well is that it's Canon's offer in the 50mm range and it's affordable. The real quality of the product matters only to a little extent, since most buyers do not make a lot research ahead of purchasing. Buying a Canon lens for your Canon camera is a very trivial and uninteresting event; preferring a Sigma lens for your Canon camera is instead a choice with significant implications.
Anyway this isn't even a criticism to the Canon system, since 3rd party offering are part of, and not against, the Canon system.