My point is that in MP players wouldn't even have to go above this limit because if instead of competing with each other key players would make a deal then they wouldn't need this whole bunch of ships at all.Well you control 1/3 of the world so yes of course, you (and European trade powers) should be making cash
That's my point: the AI doesn't go above his naval limit so all this money would be not only lower overall, but also spread around more. That said, I don't really care about SP since everyone can play however they wish and enjoy the game their way. I respect the supply limit when I play, even as a trade power, but who cares, it's my SP game (or yours, etc).
In MP however, that's the issue. There is a reason naval supply exist and obviously currently, it's not working properly in relation to some trade settings (very specific I agree). I don't see why anyone would find any issue with being over supply be such a crazy a cash sink that nobody would want to do it unless VERY desperate and for very short periods.
Keep in mind that for 90% of Nippon-Europe route European countries have exactly the same interest - move trade forward in specific direction, only competition is in Europe and if they make a deal to split the trade then they don't need much trade ships there.