SerenityOS is licenced under BSD-2-Clause License while SkyOS was entirely proprietary (and therefore disappeared when its development stalled), so the comparison seems irrelevant (your point would be more relevant if you were mentioning Syllable instead, which was quited advanced and open-source, yet somewhat seems to have disappeared... https://en.wikipedia.org/wiki/Syllable_Desktop . But since source-code is available anyone could restart it :)

I also hope that SerenityOS has a long and successful lifespan, as it seems promising !)