The game already has a growth rate for each player. That rate just isn't shown to you as a single number because it is affected by many factors and therefore always changing. This is similar to how, when you play the game normally, PA isn't shown to you, either (well, at least not in a direct way).
The primary problem with all of your arguments is that you are arguing about a number that is not available in the main game. You're having an argument about a value that only people who use the Editor would even understand. Whereas in the main game, the game makes it clear that the PA of each player is not exactly known, and even if it is known, it may not be reached. This is why the game shows you stars instead of a number, and why there is CA as well as PA.
The second problem with your argument is that you don't seem to truly understand what PA means. This is weird, because my guess is you understand exactly what the word "potential" means, yet you don't understand what Potential Ability means. As far as I can tell, you understand PA better now than when you first posted. But the fact that you are trying to advance a Growth Rate idea shows that you really don't understand what PA actually is. PA is completely different issue from PG (or GR as you sometimes call it) and talking about PG does nothing to explain why PA should not exist. Even though, yes, they are both topics that are related to a player's ability. Even players that have a high growth rate still have a limit. What you're really attempting to say, when you talk about PG, is that you think PA should be dynamic. But you've already been given reasons by others for why dynamic PA doesn't work and you just haven't truly accepted them. If Messi didn't have a limit, then he would be scoring 200 goals a season by now.
Once you can address these two problems in your argument, only then will you understand why static PA hasn't been removed in years and never will be.
If you want to have a discussion about whether FM accurately models player development over time, that's valid. For example, you can argue about how Jamie Vardy's unusual development could be replicated in the game. Or maybe you can argue that 35-year-olds (Messi, Benzema, Lewandowski) are deteriorating too quickly in game whereas in real life they are shining.
But you cannot argue that players don't have limits. That argument is doomed to fail, because it contradicts real life. It is wrong. In real life, players have limits, and that is why clubs spend 100 million to buy that special player from another club even though they already have 50 players of their own.