So i have two reasons i don't want this.
1. I don't want the developers to divert resources from making the AI as good as possible. I believe that most of us have a worry that the AI will not be good enough to pose a meaningful challenge, and having the people that work on the AI put hours into making a version of the AI that is worse instead of putting those hours into making the general AI better sounds like a bad choice for everyone. Even if you talk about diminishing achievements as you argue, i would think that having the opponent AI improved is far more important. Maybe you don't realize this but if the developer were to go down this route it would mean maintaining two versions of AI for the rest of the games lifetime, it would have to get work put into it not only for every expansion but every time they tweak the numbers of the game. I guess they could also let it become basically useless over time as well but i really want this mechanic so lets not go that way.
2. One of the reasons i wont play Victoria 3 anymore is that it becomes an absolute slog to build buildings after about half the game, not having to make thousands of clicks and instead just have the AI take on something that has just become a chore would be wonderful for that game, but if the AI is not competent as it should be i would feel forced to continue to do it on my own and that just forces a negative gameplay experience on me for no reason. So what i imagine it will be like in EU5 is that i will start out my games doing everything on my own, but over time my nation will become to large, rich or hold to much trade influence that i will feel overworked with micromanaging it all and it will be a relief to hand it over to an AI so that i can actually get on with playing the game and not having it paused for 10 minutes on every month tick.