> there's no real good excuse for the continued mainstream momentum of Debian/Ubuntu
Debian's momentum is due to the fact that it's a really stable, really well-thought-out, sysadmin-friendly, production-quality, freedom-respecting, privacy-respecting Linux distro; I don't think that has changed, and I don't think that anything beats them by those metrics.
Ubuntu's momentum is due to the fact that it supports proprietary hardware, is easy to use, and is pretty. You may be right that other distros are better than it there.
Debian's momentum is due to the fact that it's a really stable, really well-thought-out, sysadmin-friendly, production-quality, freedom-respecting, privacy-respecting Linux distro; I don't think that has changed, and I don't think that anything beats them by those metrics.
Ubuntu's momentum is due to the fact that it supports proprietary hardware, is easy to use, and is pretty. You may be right that other distros are better than it there.
Debian & Ubuntu are very different.