when microsoft flight is released you have to see first how it performs on current hardware configurations.

I remember FS9 was a hog on the pc's when it came out with very low frame rates.
When FSX came out, the same thing happened.

Flightsimulators from Microsoft have imo the intention to be run fine on future hardware configurations and have performance issues on the today's hardware.

I myself spend alot of money on addons for FSX, so for now i doubt i will move to a new flightsim. I finaly have a decent frame rate in FSX with some tweaks and like to fly in it.
With my case above, i bet alot of FS9 users have/had spend a huge amount of money to pimp up their FS9 installation and if there is no backwards compatibility, i doubt many will jump over to the new flightsim. FS9 didn't jump to FSX, so why should they jump to Microsoft Flight.