In practice it does. There's a big, entrenched competitor out there already, so that puts a damper on any motivation. If the site just closed down, that wouldn't be the case.

Even if SteamDB is gradually enshittified, there will still be that competitor that does a "good enough" job for a lot of people, meaning the bar for a new player to come along is higher.

Why would network externalities not apply here?

They’re a very common and well demonstrated pattern that plays out with this sort of site; e.g. it’s taken almost a decade for many gaming wikis to get off fandom after they were locked in. There’s a finite pool of user attention and contribution that these projects can pull from, and splitting that pool worsens the commons.