If the fictional new start-up league is going to be retractable roof or domed stadiums no way they are going to be less than $300m. And I don't think local gov'ts would be willing to foot any part of the bill. But I agree with you on the rest.
That MLS is mainly a gate receipt league is the reason it can be challenged. Anyone else who can draw well can currently compete with MLS and not even lose much money doing it. Once/if TV money starts to dominate that makes competing with MLS far more expensive and increases risk dramatically.
But you wouldn't build them immediately, you'd do what MLS did. Start out in NFL/MLB stadiums and build from there. In the midwest/northeast alone you could place teams in St. Louis, Indianapolis, Milwaukee, Detroit, and maybe even Syracuse ...No one is saying its the be all end all but its a similar model to what MLS used in its early years. Again, this is completely hypothetical and in no way do I think it would happen but its the model that makes it at least remotely possible.
No. In fact I've been clear that simply repeating yourself isn't useful. But, remember, you said "European", not me.