Outside of "for money reasons", why was the castle system not designed as an account wide system? The diamonds you buy are account wide so why aren't the benefits? Other games have Legacy / Renown / etc systems where you unlock things account wide. I'd like to start another city on another world but I don't want to have to choose which city to spend my $$$ in when buying diamonds.
A) For the umpteenth time I find myself stating the simple and obvious fact that
this isn't "other games".
B) The Castle points are based on things you do in the individual worlds, not account-wide.
C) Some of the benefits/rewards are buildings. How would you share a single building account-wide? It doesn't work.
D) As
@Sharmon the Impaler points out, the abuse possible by making the Castle system account-wide is massive.
E) Probably the only possible justification for making it account-wide is the fact that buying Diamonds garners Castle points and Diamonds can be used account-wide, but the fact that you can only use Diamonds once and not once per world kind of negates that.
F) As far as your last sentence, that's the worst justification for any change to the game, let alone such a massive change as you propose. Make your choices and then live with them. Like most of us have to do, both in the game and in real life.
As a side note, I really don't like that buying Diamonds earns Castle points. It's one of the worst decisions they've made, and the only real justification for it is the money factor. It was strictly a revenue-driven decision.