There is another thread going now that I think correctly identifies the core problem with partition: the fact that domain is everything. Because of this a partition the player doesn’t game is just an absolutely enormous setback. So there’s a huge incentive to game it, which then in turn makes the game far too easy.
If power was more derived from vassals, and in particular from internal friendships and alliances, then partition could be fine. Sure, you might lose half your domain, but if your were friends with your brother then you might be almost as strong as before, with the potential to get stronger. Or if your dad had made sure you befriended some of the other powerful nobles.
Of course if your brother was a rival and you had no strong supporters it could get ugly. But that’s exactly the type of thing that should matter in the game and doesn’t currently.
In addition, if having vassals support you was the main way to gain power, then changes to partition to give more control to the player could work better. Currently anything giving the player more control will have one result: the player will choose to keep the good provinces in their domain, every time. No penalty in terms of vassal opinion or irrelevant rivalries could ever matter enough to change that. But if having strong vassals who strongly supported you was an important and good thing rather than just being neutral (“they won’t revolt”), it definitely could make sense to choose a fair partition in order to keep your heir’s siblings happy and ensure their support. At least, the honorable ones (again, a character and relationship driven decision, which is what the game is supposed to be about).