Yes it's unrealistic to know your heir's stats from birth, but I think it's for a good cause. Disinheriting is a rather recent mechanic compared to the game as a whole, and it was introduced precisely to give the player some agency regarding monarch point generation. Because frankly, there aren't many things more frustrating than to be saddled with a 1/1/1 for 40 years. If it's between that and a little unrealism, I'll take unrealism every day of the week.
Also, I think the 1/1/1 for 40 years is vastly exaggerated in how painful it
actually is. When the game launched, bad kings/heirs were terrible because they were the primary source of mana.
These days, you have advisors all the way up to level 5 (at 50% reduced cost with some luck, some even at 75% through missions etc), you have +1/+1/+1 from estates, you have +1/+1/+1 from power projection, you have humiliate rival wars for +100/+100/+100 as a lump sum of mana, you don't have to full core everything you conquer but can do with territorial cores/trade companies, you can vassal feed with transfer province button, missions that increase ruler stats permanently, and so on and so forth.
I think we've reached a point where a) you just get too much mana, and b) the average ruler stat distribution is just way too high.
Is concealing heir stats until they're adult/takes over rulership the solution? I don't know. I think narrowing the stat distribution might be a place to start (1-5 or even 1-4) or restricting how much/often you can disinherit (maybe up the cost to -100 prestige).
Not knowing what stats your heirs have would just be super annoying to me.
