Iron Fist was in development as an MCU film early on but that got dropped pretty quick. It is true that some of the most interesting thing about Iron Fist is Kun Lun and creating a unique city takes a budget. But I don't think it's out of their reach to do. They can build sets, find locations near NYC that can double for China, have only a handful of establishing shots with some green screen. The problem has been to justify it you have to spend a substantial amount of time in Kun Lun. I think they could pull of 2-3 episodes of a season being set in Kun Lun. For season 1 they needed to establish Iron Fist in NYC for the Defenders. Season 2 needed to explore the fallout from both season 1 and Defenders, hopefully they now have freedom to expand a little beyond NYC. I hope they see a positive reaction from the final scene to give them the courage to do so.
I agree that Iron Fist may have been best served as a film (Marvel Studios can pretty much do no wrong) but I feel the rest of the shows perfectly fit netflix and Luke Cage gotta be easily accessible for Iron Fist. Where one is, the other must go and Luke is a perfect fit for Netflix. Plus the current direction of Marvel Studios these characters would not get films, which is why they're on Netflix in the first place.
The problem as I see it is not platform but execution. Every Netflix series has their own tone and style while maintaining a sense of a consistent universe. With all of the improvements season 2 had the show still lacks a style to call it's own. Daredevil is a gritty vigilante tale, Jessica Jones is a noir mystery, Luke Cage is has a blaxploitation tone with wonderful use of music, and Punisher is a down and dirty military thriller. Iron Fist has nothing, it's what you get when you strip away the tone and style that defines the other shows, it's neutral. A great delivery can make up quite a bit for a mediocre plot. Their opening credits perfectly demonstrates it. They need to lean into the things makes it stand out. An optimistic hero, use of classic kung fu films for inspiration, dive into the mysticism of the mythology. Take the premise and run with it, it's been proven that works time and time again.