I:R isn't totally a state based game, characters are present. They could very much have at least some CK2 type stuff present to avoid the constant mana nonsense. There's also a difference between not being guaranteed a competent heir and your heir not changing at all from the moment they're born to when they're a 30 year old adult. If you get a 0/0/0 heir in EU4 you're just boned, if you get a bad heir in CK2 you can try to at least get them a good education to try and salvage things. Randomness is fine but not in massive amounts.
That's what advisors are for in the case of EU4 for example. If your heir is incompetent you can surround him with very competent advisors and it'll help keep things more stable and efficient. It's not like you can't do anything about an incompetent heir in EU4.
I think the issue here all of us are trying to tailor our expectations out of Rome to our favorite game. I understand that you really like CK2 but this isn't really a sequel to it. It's so easy for us to criticize things but if we were to become developers for one day you'll see how hard it is to balance things out and make them interesting. When they're thinking about rulers they have to conceptualize it in a way that works well with Imperator features not CK2. That is why I prefer to respect the discretion of the developers because they're more aware of the overall needs of the current game design than we are.