No existing project should break unless its developers take the action to upgrade (bumping the version of the library that the library manager is looking for).

No existing project should break unless its developers take the action to upgrade (bumping the version of the library that the library manager is looking for). The reason I'd personally prefer package 2.0.0 rather than package2 1.0.0 is because a lot of tools understand the former. For example tools can let the user know a breaking update is available, or allow me to specify that my project works with many major releases of package, or upgrade/downgrade more conveniently.