[moe]

How could you make a destructive change?

Well, the same way that everyone else does it: by versioning.

The simplest approach would be to have /bin/bash2, /bin/bash3 etc., with /bin/bash defaulting to the last (current) unversioned bash release.

Then when you write a script that depends on a new feature you'd reference #!/bin/bash3 or whatever. More complex schemes are possible but probably not needed here. It's not rocket science.

And as your example illustrates: This kind of versioning is actually needed even in the current (almost stale) development-mode of bash. Its absence is the reason why many people still target sh as the lowest common denominator...

[veyron]

After some thought, what i'd like to see is what's done now with globstar (make new features accessible via shopt)