There are disadvantages. e.g. if you don't want to update a program (maybe the new version breaks your modded setup), too bad. Or if you need Windows still for compatibility, it no longer supports Windows 7, so you have to go hunting for old versions of the client and fiddle with it to prevent updates (if that still even works), at which point you'd might as well just mod it to remove the DRM instead.
Basically, it creates a failure point for setups that should otherwise last and be stable several more decades.
Not that I see. The publisher can add old versions as a "beta" that you can select if they want (e.g. Kerbal Space Program and Factorio do this), but otherwise you can only run the latest. Your choices for updating are "when launching", "when Steam decides", and "immediately". There is no "when I decide" or "never". e.g. Bethesda has apparently broken Fallout 4 multiple times over the last 1.5 years with no ability to revert after it had been stable for 5 years.
Basically, it creates a failure point for setups that should otherwise last and be stable several more decades.