It was a contract requirement from Microsoft on Asobo. Asobo would not have included it given their druthers. After I pointed out their discussions and comparisons with X-Plane in the Flight Model Description in the SDK, they scrubbed a lot of interesting information out, which is what my comments are based on, but, basically, Microsoft required them by contract to include a mode which allowed FSX aircraft to act the same in MSFS as in FSX. Of course, we’ve all seen that this didn’t work out so well.
So, the answer is yes, I believe the intention was to support port-overs. Unfortunately, there’s more to an aircraft than its flight model, and the conversion of cockpit gauges etc has never worked well and was basically abandoned.
They should just remove the Legacy flight model option. It’s silly. Or move it to the developer mode if it’s really helpful. I have no idea why they continue to keep it facing regular users.
Regarding BET, MSFS’s flight model is light years ahead of X-Plane’s, and is in fact quite similar in basis, but allows many, many more surfaces to be involved in the calculations of lift and drag than X-plane does (I forget off the top of my head, over 1,000 vs 10, something like that). Unfortunately, the X-plane comparisons have been removed, but if you can find an old copy of the SDK and the description of the Flight Model, you can see what I mean.
And, yes, Asobo is not done yet with the flight model. There’s a huge amount of extensibility built into it and there is a long list of features they have yet to implement. And to those who claim Asobo isn’t interested in Aviation, have you watched any of the developer Q&A’s? They’re all pilots and at least Jorg (and Seb?) I believe owns a plane.