Also: who is to say that Apple, Google or Microsoft manage to keep their keys secret? Not all thieves would be stupid enough to tell, and nationstates tend to hold such advantages on ice until they have a good enough reason to use them.
And as some lower-hanging fruit: The repos of common programming languages and things like Docker Hub.
Python PIP, NodeJS NPM, Ruby Gems, we pull in a lot of stuff from people we don't even know. Every python project installs a gazillion of stuff from its requirements.txt. At least the OS updates come from a party we at least chose to do business with.
And it's not like this is not yet happening already. But I think it'll take a major Wannacry event before we'll stop doing this because it's just so damn handy.
But if you think of it, imagine you're coding and some random 'willywonka2586' on a public slack group says "Hey I wrote a handy library for that, here, go and install it and use it in a project for your customers!". This is kinda what we're doing.
Was it a Microsoft dependency?
At worst the entire industry in that region is broken more often than they are functional. It's also notable that companies existing in this state of brokenness would be competing with companies living in a functional world. One might find that the defective universe continues long enough its inhabitants are extinct.