Leetnor, my thought is (and I think the other people who want air/seat transport) that you only get to have one “big” airport. It would sort of act as a regular industry (i.e. oil refinery). Obviously you have to build it, think of it as an investment for your business (someone has to transport those goods). You build one allowed international airport/seaport and that’s it. It generates passengers/cargo and if the demand is high enough (aka good connection) after a while it grows a bit, as it produces more output it needs another runway or a terminal (but obviously let’s not go Schiphol crazy, just a little bit :D), the same way current industries work in TF.
The mentioned small airfields, you could have ’em as much as you want. In this case, as a business man, you would have to decide, are they really worth it. They are clearly not needed for connecting two cities on even terrain, but what about towns up in the hills, where train track are a bit of a hustle to lay down. You decide what’s best for your business. Same goes for the boats.
About how it would impact economics. Personally I don’t really care about economics, I just want to play trains (and if my beloved planes are there even better). I also don’t think it would really have a large impact. Like I’ve said, big airport/seaport act like an industry, they just produce goods and you get to transport them. Same ticket fees apply, upkeep, etc., like we have now (judging by those beta vids. Really wish this game had an early access…).
However there’s another thing – devs. At this point it’s all up to them, do they want it, do they see all this working together, etc.