I don't think the analogy works. Windows is a closed system. The Internet, supposedly, is not. If anything, it has become a necessity and a human right. Windows is not.
DOpus is trying to replace Explorer, a sub-feature of Windows. Pale Moon is not trying to replace anything. Not Chrome, Firefox or any browser. It can't and doesn't want to. It is an alternative Web Browser, fully compliant to established web standards.
Microsoft owns Windows. No one "owns" the Internet. Well, Google sort of does with our help.
And yes, expecting every website developer to test against the 5-6 available, updated and compliant browser engines (not web browsers) is far more reasonable, than expecting community-driven, small teams of web engine/browser developers to test against every website on the planet.
And I think your minor/major, i.e. weak/strong mindset is problematic here. I repeat, the Internet is supposedly free for all. It should not be driven by large, for-profit corporations.
Microsoft can do whatever they want with Windows. Google should not be allowed to control the Internet. And by using mostly Chrome-compatible platforms, that is exactly what happens.