It is easier to see what Microsoft is doing if you look at their Xbox strategy.
And Game Pass is the new business model.
Put simply, it is cheaper than making new games to make a game then upgrade it regularly with GOTY and Anniversary editions to bring subscription folks back to play it again like it is new content.
So, MSFS, much like Halo Infinite, will see new content and support for years longer than we were used to before. But we will also see fewer true first party produced sequels per title now that the Game Pass model is driving the moneymaking strategy.
Ten years? I expect full online server support at LEAST for that time. But likely we will see a slowdown in content updates as the next console generation nears and MSFS 2028 or 2030 (or whatever) is developed.
So as long as the Series consoles are king, I expect MSFS will continue to see updates like we have been getting. At the end of that cycle, we will see Asobo (or whoever MS contracts) allocate manpower towards the next flight sim project, and updates for 2020 will likely start to slow down.