I don’t see, why there should be a problem between guys that want boats and airplanes and airports, even multiple big ones and those, who don’t want all this:
Just make it as expansion DLC and if you buy it, you have it and if not, then not.
And, if one day comes multiplayer, make it an option to turn it off and all are good to go.
There’s no need for the devs to force themselves to go one path down or another, just make both and make them optional and its ok.