How do you know?
It certainly didn't sell as well as they thought it would. You can ~speculate~ that it's because it was a medicore game (I haven't played it, but by all accounts, you are not wrong) - but most games (not all, just most) have the majority of there sales near release, and being a Day 1 Game Pass game certainly helped contribute to a loss of sales. But I couldn't say by how much. And unless you have some sort of insider info, neither can you.
I could speculate:
At some point, Starfield at 17 million concurrent players
It sold about 3.7 million copies on Steam.
In the first week, it sold about 150,000 copies on Playstation (no idea about lifetime)
And supposedly it sold about 1 million Xbox copies.
So... add that up and subtract from 17,000,000, and realize that concurrent players is very different from units sold.. and it leads me to believe that Game Pass had some significant play for Starfield
Now - even with all that data, there's no telling how many of those Game Pass players really were a lost sale - many could just be checking it out because they had access to it. Or how many people signed up for Game Pass just for access to Starfield.