>You can go to SteamDB.info and learn a lot about millions of people based on what games they buy. It's a psuedo-anonymous per-user purchase history. Here's mine: https://steamdb.info/calculator/76561197975362423/?cc=us . You can look up basically anyone on Steam, and even do advanced math on all of it. It's been going on for at least a decade.
Sure, but if you game this out, it still comes down to whether or not your opinion is that this data should be public. That Steam's methodology happens to be opt-out sharing - in other words their opinion is it should be shared, because people don't change defaults - doesn't change that there haven't been any known major harms and many major benefits, especially in the opinions of game developers.
can't you opt out by making your profile private?