Pure class balance is only ‘possible’ depending on what your definition of balance is.
The game doesn’t need to be ‘perfectly balanced’, just ‘balanced enough’ to the status quo of players.
Balance in terms of what? PVE? PVP? What about the different combinations of classes and group comps you can bring to an encounter or match? You’ll always have some astronomical number of permutations that the only way to ‘balance’ the classes would be to homogenize the kits and output classes bring. That can be bring out an set of gambits people will use, but one style will be reacted to another to the point that it reaches boredom.
Class balance will ALWAYS be shifting due to the simple fact that player tastes change with the shifting of power and addition of new abilities added to classes. No class or spec plays as popular forever. If a class is weak and gets buffed, people flock to that and FOTM changes the meta. It creates a feedback loop that keeps the game in constant flux due to people adapting to changes made to the game and then the developers making adaptations based on player responses.
If there’s something egregious enough and enough of the players paying the bills take issue with it, the changes will be made based on that. People need to change their expectations as to what balances will be made to the game.