Doesn't have to be on DVD. You can make the backups on HD if you want.
If it's backed up on your hard drive, wouldn't it already constitute being already installed? Even if it's an external drive, you can play straight off of it.
Remember, this is in some hypothetical situation in a future where things happened that won't happen, not the real world.
Nobody knows what will happen in the future with any certainty.
Here's an example scenario: Gabe Newell dies of a heart attack, and Valve is sold off. Ubisoft purchases it, proceeds to run Steam straight into the ground, people leave in droves and eventually Ubisoft kills Steam because it's no longer profitable.
All of that could happen in a span of five years, perhaps even less. And it's only one of a myriad of ways in which Steam shuts down; other scenarios might take longer, but the end result is the same. And what if Steam doesn't shut down, but simply became so much of a nuisance that you don't want to use it? Not even their promise to unlock all your games applies anymore, since Steam is still alive.
You might be willing to accept the risk of losing access to your games, but that doesn't hold true for everyone. Losing one game is unfortunate, two or three regrettable, but we're talking about accounts that may have hundreds of games; what's the value of that? Thousands of dollars' worth of goods, ready to disappear at a moment's notice. Are you willing to accept the risk of losing all that money, plus more for the games you have to repurchase? Not everyone can afford to do so.
Obtaining a copy of a game that you have a legitimate license to use is not piracy.
You would also only have to resort to this if 2k completely abandoned the game and refused to provide an alternative download location.
So yes, if an extremely successful company went bust and you'd uninstalled the game and not backed up the game data and the developers of the game also went bust or refused to support it then you would still get to play the game as long as you could obtain the games data from any backup on the planet.
I'm sorry to inform you that if the internet also burns down you may have difficult locating a copy of it if you do not have any friends who also own the game and can back their copy up to a CD for you.
Otherwise you are going to be doomed to a lonely internet free existence without any Civ.
Do you think we should start planning for this eventuality right now?
And where is this license? Right, it was with Steam. My proof of purchase is a credit card bill from a decade ago. 2K games offers an alternate download, if I can provide some kind of proof that I actually did purchase the game, except I never saved any of it because I didn't think it was necessary.
Of course, the license you purchased may or may not have a provision regarding the usage of backups (when was the last time you even looked at the license agreement?), and there's reasonable doubt as to whether or not a pirated version even qualifies as a backup to begin with, since you're not the one who created it.
These are very murky legal waters to be dealing in, yet nobody seems to care simply because they decided that the convenience of digital distribution is worth sacrificing the physical copy and all the rights it entails.
If your answer is to download that pirated version anyways because you know you paid for it, then you had best hope that you don't end up in court for any reason, because you'll be paying far more than the fifty bucks for a new game.