I agree in being more attractive I disagree with the stick idea you had. I think the carrot would be better.The thread was meant to imply that having vassals should be made way more attractive than it actually is.
Your Original post encourages the player to go vassals by threatening the player with no levies. This encourages players trying to maximize levies to exploit something else. Unless a carrot is also provided another thing is simply going to be exploited.
Honestly I would be some what ok with levee having demneshing returns going over demise limit. If their was also a way to boost levees gotten from vassals. For example maybe having martial trained rulers able to increase the levy % given from vassls based on the martial stat with default 0 bonus being 10 in the stat so the strong martial rulers generally have stronger armies. This discourages NK mode because vassals then start giving more troops then default NK would provide.