I don't think so, it'll be exclusive on Microsoft Store just like what they did with Forza Motorsport/Forza Horizon. For some people (including me), back in the days where Microsoft Store was buggy as hell, I downloaded Forza Horizon 4 times its size (almost 400GB).
They announced that MSFS020 will be cloud based game and will use subscription scheme instead of one time purchase. Prepare your heavy $$$$ for this one