Sea routes ARE changes to the world because they connect place A to place B. Basically, you could just as well change the map so that A and B touch each other, that's the effect that sea routes have.
And that is why sea routes suck, because they connect A to B, but not A2 to B, even though you could make the same argument for A2 as you made for A. So either it is tons of sea routes all over the place, which makes everything connect to everything and makes coastal cities totally vulnerable towards naval invasions, or you don't.
All you guys arguing historically are forgetting an important detail: There are no actual sea routes in this game. Nowhere. We simulate them by having connections between unconnected regions, but the game doesn't know these are sea routes. It doesn't know that landing your soldiers in an enemy city by sea is not the same as marching in by land. It doesn't know that you could be attacked by an enemy navy at sea. It doesn't know about weather, it doesn't know about ports, it doesn't know about the fact that sea routes should be limited by the number of available ships, it knows bugger all.
And that's just the tip of the iceberg.