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 imposed on them.
Ever hear the saying "If you owe the bank 1 million dollars it's your problem. If you owe the bank 10 billion dollars it's their problem".
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".
Is this anything other than a problem the big 3 have made for themselves for what are essentially marketing reasons?
Is codeshare as a "solution" really the best solution or is it just the best solution that also meets the marketing teams requirements?
Do smaller airlines even care for a solution? Wouldn't a global industry wide solution basically be the big 3 bullying the smaller players to spend tons of money so the big 3 can juice their own mindshare?
This is a problem for people inside those 3 airlines to solve within the constraints that the current living world has imposed on them.