My question is, is it worth buying an X-Box one / series only for the upcoming Microsoft Flight Simulator 2020?
I have a PS4 and actually only play there.
In my opinion, it's not worth it because of a game.
Wait and see how well the game is received by the people and then think about it
So I use X-Plane 11 on a gaming PC and can only say that it is much better than MS X. I would wait until MS 2020 comes out and then ask in relevant forums what it brings, now I would not buy an XBox especially for that.
But your money and your decision.
If you only want to get an xbox for MS2020, I'd rather buy a pc right away, on the PC it will be a lot more detailed and extensive in terms of the key assignment, etc. Than on xbox.