By my very humble opinion:
There isn't a term limit in the game because there wasn't one on real-life kings as well. However, real-life kings have a built-in term limit, it's called life expectancy. It is something (aging) that I brought more strongly into Might & Fealty because I felt like I hadn't done it right in BattleMaster.
I would strongly support any idea to increase ruler turnover without making too hard rules about it. But our players being who they are, they will quickly find ways around any restrictions. So the driving force behind should be PLAYERS.
And in any government system, if the players want to change their ruler, they can. So if someone rules for 10 years, apparently most players in the realm are happy with that, or they would've thrown him out.
