The "quality" of an OS is a subjective thing. Some people may find it important that and OS is as secure as possible. For others, software availability is king. Some find it important that it's pretty or easy to use. I myself am very ideological and go for freedom, anything that avoids any kind of hard or soft vendor lock-in.

So, saying that the quality of an OS is clearly also determinded by its eco-system as as wrong as the word "clearly" in this context suggests. I assume it's an important thing for you, otherwise you wouldn't have said so. But it's maybe not for others.

The GNU/Linux OS and its various distributions has found its niches and roles where its quality (quite obviously) is considered better than that of one of the alternatives. In the end, that's what's important: That those who need what Debian/Ubuntu/Fedora/... have to offer can use the respective distro and don't have to resort to Windows/OSX as the second best choice.