Frankly I've never found a Linux variant I wanted to keep and use everyday.
And I'll admit that MorphOS is not a main stream OS, but with about 1% of the market, neither is Linux (and not only am I willing to leave MorphOS installed, I use it and am willing to pay for it).
What makes an OS mainstream? Commercial adoption? OS X has maybe 4% desktop adoption, does that make it a viable mainstream OS?
Does developer support make an OS mainstream? Linux has TONS of developer support but not many adopters?
Please quantify your statement about mainstream so I can better understand where you're coming from.