Not entirely true. After all, you can do an awful lot to increase the odds of a male heir dying via generalship, allowing you to try to roll up a new heir.
What's much worse is regencies. Oh, your king fell down a flight of stairs and his third-cousin, twice-removed just had a child that he swears is a valid heir? Enjoy your 15 year regency (with stats rolled at 2d4-3) and ensuing non-existent legitimacy, since you can't just smother the brat and let a local (or foreign) noble take the throne and get back to normal affairs because, for no good reason, regents are incapable of declaring war.
Regencies suck all the fun out of everything for as long as they last.
I couldn't agree more on the cant declare war part, it feels unnecessary, regencys would feel pretty ok without it. It is limiting the sandbox too..