When we talk about Linux as an OS, we don't talk about Linux as the kernel. This is very tiring to read this everytime. The Android userland has nothing to do with any of the Linux distros out there.
It's not Linux who went mainstream, it's Android userland. Try running Android applications in "non-Android" Linux for good fun.
GNU/Linux + X11 + Gnome/KDE never went anywhere near mainstream, perhaps, but Linux sure as hell did.