Except virtually everyone got it wrong still. Even the head of Larian thought it'd top out at 100k max. That's currently it's average now with it's max being more than 800% higher.
BG is a big IP, but it's never had this level of success. Look at Diablo III's release (similar IP with a long break between games). It had better advertising campaign and still kind of became noise fairly quickly. Game news sites barely covered BG3 until it hit it big.
Microsoft definitely undershot, but it was likely basing it on a lot of the aggregated news as well. It had barely any coverage prior to its official release. This is usually a sign that the game will be mediocre.
Larian is a big studio but its last expected game from its really only known IP was cancelled after being put on hold for four years (granted BG3 was also being developed during this time). It's biggest games prior to this got at least partially funded on Kickstarter (not a knock against KS, but it's not generally seen as the sign of a strong studio to exec-types).
I don't blame an executive for not seeing this coming.
Executives obviously didn't see this coming. But neither did game journalists or even gamers.
Its a mistake in hindsight, but with what everyone generally knew at the time, it was the expectation of most.