It is right that you don't need to build sturctures that often. But decreasing the amount of income would also mean a deacrease of armies, which would make the game more static and you would have big problems, if you are attacked by three opponents at the same time (which happens quite often). At the moment you can just stop building in order to create more armies. With this decrease you would be just outnumbered without the chance to defend yourself proberly.

Of course this is just a guess, but I would suggest to not make this cutback as you have in most cases in the 2 year per turn enough money problems.