My flip-flop is that this shouldn't happen. As Oleg pointed out earlier, any
system in Olympia without decay will evetually lead to the system reaching
maximum capacity. That means a castle in every province, ten ships at every
seaport, and a superhighway between every wide place in the path.
So, we're back to a decay idea. Ships, Castles, and roads must require upkeep
(whether you need to throw money at it, or simply labor isn't important), and
this upkeep should be pretty heavy - a castle should require possession of
several provinces to support it, a ship requires booty or other cash stream.
If it's not monetary directly, the labor force will cost a good hunk of change.
So, I remove my objection to Rob's system, as long as decay is a strong force
to fight against, not just a weekend a year fixit.