You don't need to update Steam to play TF2 any more than you need to update Gnome to play TF2. By your reasoning, if you should need to update Steam to play TF2, and therefore need to keep Steam out of the package manager, you should also update Gnome, and therefore keep it out of the package manager, as well as the kernel, and pretty much every other package, just to play TF2.
Ergo, "you should discard the package manager entirely".
Steam is separate from the games, it's just an installer. You can play (and update) the games without the installer, therefore at least Steam should be a managed package.