I think that's more due to player competence than fundamental easiness of dynasties to survive, though. AI controlled dynasties that started large go extinct frequently, and in the course of a game the player is liable to personally be responsible for dozens of small dynasties going extinct. It's just that players are usually smarter about marriages, utilize intrigue better, lose fewer wars, and rarely lose all of their territory (unless they're trying for a masochistic start). The AI also doesn't care as much about having an out-of-dynasty heir, given that players are supposed to do everything they can to keep it from happening.
I would say it's very difficult to have a game be unbalanced enough against the player to be compelling without resorting to ham-fisted restrictions that create more frustration. And if you want to have players and AI play by the same rules, which most games don't even attempt, it's more difficult to keep that balance between the extremes of player domination and frustration. In EUIV in particular, my enjoyment is more about finding a starting nation that provides a challenge and goal, and having the mechanics provide a way of meeting it without making the whole thing trivial. And for the most part, it works, but they do mess up really often with balance changes. Thankfully modding is extremely easy.