Android is too bad to be Linux. It's a platform dominated by closed-source application software, a cathedral-developed kernel and core, and dubiously open bootloaders. It's utterly useless for development, and it makes numerous sacrifices on many levels to try to appeal to 'regular users'. When one pines for Linux success, one means GNU/Linux [and whatever DE or graphical environment you like]. Nobody cares what kernel your phone runs.
All fair points, but it depends on how you determine success. If you measure Linux-based OS'es penetration in the mobile market today, you will find that it 's the biggest player. That's success.
Yes, you have problems with locked bootloaders and closed hardware on some phones, American carrier-phones in particular. This is bad, but no fault of Android.
That most software on Android, a Linux-distro, may be closed source may sadden a FOSS proponent, but it still doesn't mean it's not Linux.
On my Android tablet I can fire up a terminal, hook up a keyboard via USB and then hack away in a Linux userland, using either supplied binaries, or busybox, or other Linux binaries compiled for the ARM architecture. And it will all work.
For lots of tasks where in the past I would need a PC, I no longer do. Because my Linux-based, mobile platform has me covered. If I want to build my own stuff, I can actually use the normal Linux toolchain to do so. I can do all that because Android, either you appreciate it or not, is Linux. There is no debating that.
And right now Android is dominating the mobile space. I think it's fair to say that Linux, in a form you appreciate or not, has succeeded where your traditional DE based Linux-environment has not.
There's a lot of closed source software for Android and many phones are sold with locked bootloaders.But it's possible to buy those without and google don't care if you root your phone.
So unless your definition is a system 100% open source software that is of no interest to "regular users" (not even Ubuntu qualifies) then this is unrealistic unless you are Richard Stallman.
Android is just too good to be Linux. That sort of reasoning.