Lol, how the hell did they expect it to do as well Fallout 4 while removing the game from a massive install base?
I think they really thought people would jump ship and buy into the XBox ecosystem or PC Game Pass. That was their thinking when they made the decision to make it exclusive:
It's a bit of a gamble from Microsoft, which clearly believes Starfield will be enough to get people to buy into Xbox and Game Pass purely for Bethesda's new game. 10 million sales is a hell of a lot to sacrifice for a game, especially one that's been in development for so long at a development studio as big as Bethesda Games.
Source.
Just yet another example of MS misreading the market and an overestimation of what current Bethesda is capable of, as well (ie. Skyrim was a long, long time ago).