I suspect that the moderately disgruntled vets could be engaged and brought back, at least, in the short-run. There are some players (Krilcov et. al.) that hold grudges.
If I was an Imm, I think with some dialogue, hearing out these moderates and a few steps in the right direction, there would be a positive outcome. This might take a little time invested but not a huge amount.
The less moderate disgruntled players would take a lot more time, expect significant fundamental changes to the game and cannot really empathise with the work that would go into it anyway. If the time could be better spent elsewhere and most players are relatively happy, I'm not going to be banging my head on the wall for the far reaches of the playerbase distribution.