Just to be clear, they aren't talking about disallowing changes that are backwards incompatible. For example, they could introduce a new 'foo' keyword that won

Just to be clear, they aren't talking about disallowing changes that are backwards incompatible. For example, they could introduce a new 'foo' keyword that won't work on previous versions. What they are saying is that they won't introduce changes that break the way already existing code works, like they did by changing the underlying behavior of existing types. I think this is a reasonable compromise that allows for software developers to get access to new features, while at the same time refactoring their code once migrated to a new runtime.