It sounds similar to the Linux kernel. If all the providers and consumers of an API are in the same repository, then you can change an API at any time as long as you update all consumers of the API at the same time.

It also results in having a huge version control repo. From what I've heard, Google can't move to git because of this... they're stuck with Perforce because it's the only VCS that can support a repo of their size.

It does discourage upgrading until you really need it. On the other hand, if one person decides to do the work then everyone benefits.

That sounds quite implausible. Sure, one person can go and upgrade the library.

But I find it hard to believe that one person can go and fix up all the projects he's hardly ever heard of that depends on that library.

We have good tools for this. A Googler can create a patch that upgrades a library and run tests for all projects to see what breaks. The project owners review the changes. Of course, some upgrades are easier than others.

In some ways this is similar to what Linux distros do, but sharing common source control, build system, and test runner makes it easier.