I'd argue we could have both if not for commercial reasons. Apple deliberately chooses to limit control, Microsoft too.
With all the resources they have, you really think they couldn't give you more control without sacrificing stability?
There isn't infinite time and money in the world. Every hour making one feature more stable means an hour not invested in another. Flexibility and control are just additional features. If you want to change the system font, you have to test (and support) a much wider array of things, especially if your UI guidelines are as restrictive as Apple's. Every piece of control you expose to the user increases the engineering cost of writing and maintaining the software.
And frankly, if more people wanted that level of control, this year TRULY would be the year of linux on the desktop. But they don't.
Control is deliberately withdrawn to preserve the commercial interests of the producer, not because it is technically impossible to add it to stability.
You want stability? Provide a perfect default configuration. You want control? Allow tweaking that configuration in detail for users that are willing to do it.