Earlier quoted context omitted.
True And that's why they should have started thinking about this 20 years ago (or even before that) Thinking of something like this: every new development and maintenance should be done with 5 digits in mind. While keeping it backwards compatible for 4 digits
There was no need to have 5 digit codes or more than 0-9 for the flight numbers 20 years ago. Arguably there isn't a need now. It's 3 airlines in the entire world which have this issue because they want to have their own 4000 flights plus codeshare more than 6000 other flights from partner companies. This is a problem for people inside those 3 airlines to solve within the constraints that the current living world has…
You list listed off the largest airlines on the planet of which pretty much every other airline has to interoperate with. This isn't "AA just has to deal with it", it becomes "This is now everyones problem".