Earlier quoted context omitted.
Agreed. These toy-features are especially bitter when you look at how little bash the language has evolved in the past decade. Pretty much every system out there drags a significant baggage of shell-scripts along. Yet there's been barely any improvement on the arcanest of all syntaxes. We keep on painstakingly reinventing ever the same control structures (half-broken lockfile mechanisms, retry loops, miserable suppor…
Lets imagine for a moment that cross-platform backwards compatibility is a high priority, and that every change leads to a frustrated group of users running scripts from many years ago. How could you make a destructive change? As an example, this works in bash 3.2 but not 4.0: diff in 3 it expands to diff whereas in 4 it expands to diff
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...