Changing major version numbers does not fix compatibility issues
So now that we know what's wrong, how do we fix it? The answer is to clean up the technical debt, to go through the code and make systematic changes to bring it up to 2014 standards.
This will fix a lot of bugs, but it will break existing shell-scripts that depend upon those bugs. That's not a problem -- that's what upping the major version number is for. [...]
There are several issues here, but let me start with the last statement. This statement is wrong, and let me say it plainly: changing your major version number does not fix compatibility problems. Changing your version number is not some magic get out of problems card that makes your new version compatible with your old version; regardless of its version number it remains just as incompatible with the old version as before. The purpose of changing version numbers is to warn people of potential problems, and by extension to reassure them that there won't be problems for minor version changes (which makes them more likely to adopt those changes).
If you introduce breaking changes in Bash or in any program, things that people have today will break. That is what breaking changes mean. People will still have to either fix them or stay with the old version of the program and yes, the latter really is an option whether or not you like it (it is especially an option if you are proposing changes in something that is not actually your program). When you introduce breaking changes you are asking people do extra work, which they do not enjoy in the least; generally the more breaking changes you introduce the more work you create. If you want your changes to be accepted, you need to be very sure that what you are offering is worth that work. Otherwise you are going to get an unpleasant surprise.
Changing the major version number does absolutely nothing to change this dynamic and thus it does nothing at all to make breaking changes less of a problem or less of an issue. You cannot hand wave away the cost of breaking changes this way, no matter how much programmers would love to do so and how this 'we'll just change major versions' thing is an ever-popular idea in languages, libraries, systems, commercial software, tools, and so on.
(Note that being left with no real option but to do a bunch of work also drastically lowers the cost of moving to some other alternative; you no longer have the difference between 'do nothing' and 'do the migration', now it is just the difference between 'fix the existing stuff' and 'do the migration'.)