I think it's not a huge point, to be honest. In my experience, in this game to have 9 or 10 stamina is pretty much the same.
However, I kind of still disagree though. I see no reason to see a decline in Stamina while training Stamina. And I don't think it was conceptualized as overtraining by the creators of the game. I think it's just how they did it, and no one really complained about it before. Of course, we are all aware that Prominent means 10.0, and Proficient means 9.9. So the supposed effects of any change here are probably slim to none.
Also, since training Stamina training gives something like 0.5 per week, the drop is probably more like < 0.1/week. I think it's more of a potential cosmetic change than anything else.
On the other hand, Stamina is a full-team type of training. So the overtraining argument is slim in that perspective. Let's say I have some "Stamina 1" guys and some "Stamina 10" guys. Now it looks like some kind of penalty for the top stamina guys