The search for the correct amount of split-lock misery

That is a somewhat odd position to take, because on Windows:

1. Steam installs a service, which runs as "Local System." This is entirely equivalent to a daemon running as root, it's just that Windows uses different terminology.
2. It is not uncommon for Steam games to display a UAC prompt on first run. This is usually to install MSVC++ redistributable or something of that nature, rather than doing some weird custom thing, but it still technically qualifies as elevated privileges.
3. The way that Steam manages to install games without requiring a UAC prompt for every installation is kind of... terrible. Basically, it sets the library as world-writable and world-executable. That means anyone can drop a DLL in the same directory as the game and silently hijack it, because Windows considers the directory permissions to constitute a security boundary (indeed, you could just as easily replace the whole exe file). Now, you may argue that this is irrelevant, since the game is running as an ordinary user rather than a superuser, but we have to consider the https://xkcd.com/1200/ factor.

Note: (3) is my understanding of how it works, but like a lot of gamers, I do something *even more terrible than the above* and install everything into C:\Games instead of Program Files, so I don't know for sure if (3) is the default behavior or just my own damn fault. Why do I do this, if I know it's terrible? Because life is too short, that's why. It is not uncommon for older games to randomly break if they don't have full write access to "everything," so deliberately subverting the OS's security is not uncommon for PC gamers. Even if Steam is doing everything right, that doesn't help when half the users are overriding the defaults and using a less-secure configuration.