Personally I would prefer a game ripped to iso format and sold on a USB stick. Where I live my Internet D/L limit makes buying more than a few games per month impossible. Otherwise I think Steam is a 'decent' service but I don't trust it completely. I think its too easy for them to change the rules and make your games unavailable. Not that has ever happened to me. But lets face it, anytime you store stuff in the cloud, anything can happen!
Aside from not requiring you to sign into offline mode once to make it work (because of encryption key passing with steam?), I think steam is the best platform for PCs. Cheaters, who ruin online play, can be banned from any VAC (steam anti-cheat) enabled servers, but no limitation of using the actual game. I don't really care if cheaters go play with other cheaters or people cheat in single player games, but playing against cheaters online should be a choice not a requirement because of the lack of safe guards. Go check out O-ther services and see if the same would even remotely happen or if their entire account is banned.
Aside from valve going under, steam will be around for a while. At least you can safely assume for the life of steam, any games using steamworks won't up and randomly quit working because the publisher has no server running anymore. I mean honestly there seems to be a better chance of a random company getting gutted, bought, bankrupt and the game becoming worthless. What happens when that $40 game doesn't work because the publisher required some 3rd party activation and their servers were taken offline as a "reorganization" or something.
Games being sold on discs seem to be shrinking aside from consoles. Best Buy seems like they are going to do away with any physical media for any software soon enough. Best Buy is pushing their digital store and EOL'ing even relatively recently released games.
I mean let's be honest, out of all the people that purchase software, how many go out of their way to get physical copies? The bottom line, the cost of doing business in the digital space is significantly less than having all the overhead with physical locations. I think sooner than later physical media will become the way of cassette tapes (I'm not that old

).
Maybe as that shift happens people's internet will become faster as more people use it more heavily. CDs transferred around ~5MB/s and DVDs around ~20MB/s on the fastest sequential speeds. When internet speeds are >= 5MB/s ~20gb games take about an hr to install, which honestly isn't that terrible.