Everyone likes to crap on C++ because it's (a) popular and (b) tries to make everyone happy with a ton of different paradigms built-in. But you can program nearly any system with it more scalably than anything else.
As for compilation times, yes that is an issue, they could have switched to Java as other Google departments were doing, with some JNI if needed.
As sidenote, Kubernetes was started in Java and only switching to Go after some Go folks joined the team and advocated for the rewrite, see related FOSDEM talk.
I do not know why they did not go with java, I imagine building a java competitor (limbo) and then being forced to use it is kind of demeaning. but again, this would all be conjecture.
That's quite debatable. C++ is well known to scale poorly.
I think people who write complex applications in more sane languages end up not having to write millions of lines of code that no one actually understands. The sane languages are more concise and don't require massive hurdles to try and bake in saftey into the codebase. Safety is baked into the language itself.