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.