Considering the actual release of the NGX and the simple fact it uses PMDG's new SDK outside of FSX SDK as it was too limited in functionality, I'd say we have seen enough to believe and the rest is common since. If knowing anything about anything you come to a quick conclusion that this is what took the most time making something from the ground up and involving high levels of C++ and new VNAV algorithms that were not carried over from past projects. And if they did not carry over old development techniques or use FS9 NG then I highly say that they will use their new development techniques on the 777 to speed up production as they are not stupid and of course would do this as the overall goal of the new SDK was not single handily for the NGX but all projects to come as it gave them the power to do things they could not do before and would not have a need to make a even newer SDK for T7. As some of the T7 project has already been worked on even in the NGX development.
As for framework this is tons of C++ code witch will only need to be altered/tweaked or variables adjusted to allow the 777 specifications. dud
I say your Flight comments regarding the T7 vs FSX lifespan are further out in left field then the 777 development and I think you should turn you attention there if you need to "see to believe" as Flight really has not shown jack or mention squat about Flight and what it's to entail or if it will even be a platform developers what to developer third party add-ons for to even talk of it replacing something. (A few videos and pics) Developers who have talked to Microsoft have signed NDA and Flight's website leaves a lot more questions then answers in just were they are taking it, let alone a add-on market and not even the same team is making it. It's likely to try to reach a larger market witch in return would make it more a game/arcade then a sim for someone well establish in the current and past MSFS franchise. Hell it could have DX11, Tessellation, or even Physics all of witch would be new to a current FS developers on top of what maybe a new game engine. Really if I were Microsoft I be pissed that my 80 dollar game has it own little sub economy behind it that they get no parts of.
And even if the T7 took three year FSX still be kicking as people still play FS9 and I expect there will still be some who have not retired FS9.
But hey what do I know? I only have 4 post here, and the all the 'experts' have hundreds.