The uncertainy is the main reason why I'm considering a switch, actually. I touched on it a little bit already, but when you're dealing with stuff that isn't tied to a service like Steam, things tend to get... a bit dicey.
I'll take Mass Effect as an example, since I mentioned BioWare earlier. If you have it on Steam, the Bring Down the Sky DLC is simply downloaded, no fuss. If you have a retail copy, there's no indication of what to do with it when you launch the game. Turn to Google, and you'll find a multitude of sites referencing dead links on various BioWare pages, like this one -
http://masseffect.bioware.com/me1/galacticcodex/bringdownthesky_pc.html. Part 2 says to go to
http://accounts.bioware.com/my_account/mepc_cdkey.html, login, and enter your key, but the link just redirects to bioware.com. From what I recall, there's nothing to indicate registration or ownership of BDtS on your account, either. You can eventually figure out where to go, but it takes a bit of time and digging, and the page itself is obviously a relic of sorts, existing only in a sort of unofficial, forgotten capacity.
Things like software being old and service servers shutting down is what's impacting physical releases, not digital ones available through services like Steam and GOG, because the method of distribution is not a stream of forgotten publisher sites.
You also have numerous games like Dead Island, Fallout: New Vegas, Dawn of War II, Skyrim, Dishonored, Borderlands 2, every Valve game, BioShock Infinite etc. that require Steam, which means the disc basically serves no purpose - you can't play the game without activating it on Steam, and you don't even need the disc, since you can just enter the code on Steam and download the game and all relevant patches more quickly.
The key difference here with consoles is that your console is the DRM. It can't be played absent that specific hardware, outside of emulators, and those take years to reach maturity. Extra authentication crap is possible, mainly with companies like EA (notice a trend here with those assholes?), but offline, disc-only play is generally still a possibility... at least for now.
Since computers utilize a massive variety of different hardware, which is often upgraded, the system itself can't be an authentication system. Some companies have tried this, by tying activations to hardware, but apart from a small number of exceptions, this is again more of an issue with physical releases than with digital, because companies are using digital distribution services as a means of authentication - like a console.
I wish things weren't going this direction, but based on some of the issues I've run across, it seems like physical PC releases actually present more problems and uncertainties about long-term use than digital ones.
