Since Linux has no concept of a base system, it's a stand-alone kernel with a hodgepodge of crap around it - this distinction makes no sense on Linux.
/opt is generally for software distros for which you don't have source; only binaries. Like commercial software packages. More common on Real UNIX(R) because most Linux users outside enterprise aren't running commercial software. You're putting your $500k EDA software under /opt.
Steam says hi.
On Windows, a common Steam library exists in Program Files directory, therefore not user specific. On Linux, each user has a separate Steam installation and library. I'm not sure why there isn't a common Steam library on Linux, but /opt would be a good place for it.
No reason this can't be done on Linux but since NT's security model is more flexible it's a lot easier to do so on Windows. You'd need to add dedicated users. (Running a Steam daemon as root would probably cause an uproar.)
Developers who knowingly reduce or disable default Windows security settings should be censured. Because in 99% of cases it is due to ignorance or plain laziness.
The actual solution, which remains both compatible and consistent with the security model, is that you should have to be administrator and pass UAC to install a game, just like you do to install anything else.