[chrisdevereux]

You're right, it's not as bad. But the metaproblem here is making the assumption that the global environment is not going to add new APIs.

In the case of Angular zones, if additional async APIs are added to the browser, Angular needs to be updated to patch them with its own zone implementation.

This wouldn't break existing sites, but could leave, for example, sites on older angular versions unable to use new browser APIs, use libraries that use new browser APIs, or install security updates to dependencies if newer versions rely on new browser APIs.