Anyone noticed how the GN tool used to build Fuchsia looks like someone really wanted to use Bazel but for whatever reason they had to use Ninja?
Fuchsia is really close to the Chromium team and inherited gn and most of the supporting repo management and build tooling.
gn is less strict than Blaze and Google probably doesn't want to maintain both Bazel and gn forever, so it stands to reason that gn will eventually be replaced with Bazel. That will be a really large effort so it's probably not happening anytime soon and will take a while.
Android, which uses yet another home grown Blaze-alike build tool - Soong - is already in the process of migrating to Bazel. Moving from Soong to Bazel is probably much easier than going from Ninja/Make to Soong due to how conceptually similar those are.
Most people use Bazelisk: https://github.com/bazelbuild/bazelisk
Blaze owns the whole thing, executes the toolchain, etc.
Blaze is also very much built for the monorepo model we had in Google3. Not that it won't work in other scenarios, but that's where it came from: one repo and when it comes to third party dependencies, one version, etc. And cross-compiling on it felt awkward. GN/Ninja support for multiple toolchains and cross-compiling felt more thought-through, by necessity.
Well, something I'll test hopefully somewhat soon, as I want to test bazel explicitly in very crosscompiling environment...
I'd argue this is still the case right now, but toolchain support in Bazel is maturing quickly.