Specifically, allowing arbitrary increases to troop equipment that just costs gold & time would be yet another way that large, wealthy realms could too easily snowball and become unstoppable.
I don't mind things being as they are, but I'm not sure I follow this argument. Small realms are usually stuck with a few !@#$ RCs, because they can't afford to constantly renew them to get good ones, while large realms can afford to have so many RCs that scrapping some more or less regularly for better ones makes sense. In other words, wealthy large realms are the ones that are most likely to already have great RCs and thus not require any means to improve their units.