A few of these referees have stepped away from Basketball for the reason of their full time working career which puts the bread on the table. They may come back in a few years time when the bread is baked.
However, there are some refs in this list that stepped away for a number of different reasons from player/coach/spectator abuse, nil support from BASA and the tribunal system (which is a very big point that needs to be resolved), little communication, some refs see the door to advancement close quicker than they want it to, and very low pay to boot.
Maybe, some of them got where they were going too quickly and walked away because they couldn't handle the toughness of the communication from all angles (i personally think that this needs to be addressed with officials to help them develop the thick skin). The good officials take in their stride and deal with it - they stick around, their love for the game is bigger than that. If this is the case, then those referees may never want to come back as they will love other things.
What BASA can control is the pay (incentive) structure, support (incl tribunal, learning and development and social aspect (a lot of past referees would go out and referee just to have a hoot of a time with their fellow referee mates.
There are many reasons why referees quit. Some will never come back because they never had it in them anyway and don't love the game or miss the game as much as they should. Others won't come back simply out of principle of how they were treated in the past.
I think, those that are lost due to the above, maybe lost for good - that is something for everyone to think about next time there is a tribunal, or next time the pay and incentives are being reviewed, or next time a know-it-all coach or spectator chases a referee in to the car park calling them a cheat etc.
So, best case scenario is to focus on the stocks that we have now and the referees coming through. Treating each one of them like your own and making it a more pleasant environment overall. Basketball from all angles needs to be a more pleasant environment for everyone.
This is no different to how a player may feel when considering when to stop playing or switch sports. How many make it back to Basketball ???