Zimerius: I've never been denied access to a game before !
Well many others have over the years.
So either you are relatively new to Steam or just been very lucky.
Many of course don't play games every day, so they could miss a servers unavailable event.
And I am not entirely sure if they have an auto-switch servers option in place, but I seem to recall from long ago, that you could change what Steam server you used. It probably wasn't all Steam servers being unavailable ... maybe just one country or a few.
In any case, if you had set your games to play Offline before it happened, then unless Steam was due for its monthly (or whatever it is) check, you should be good until they become available again. That wouldn't help those connected at that time, but it sure pays to always set to Offline after you have finished with Steam for the day.
Even if it was a planned event, probably too difficult to switch all users (could be millions) to Offline if they weren't already, before going ahead.
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As for the future.
It doesn't make a lot of helpful difference, if someone makes a fake Steam server, as you would only be able to use that with the Steam games you have already downloaded to your PC.
So if Steam were to fully go down and for an extended period or forever even, all their customers are gonna be in a very bad situation. If Steam gave some warning, and DEVs and PUBs came onboard, and someone else took over Steam's libraries, so that customers still had access, then probably not much of issue, just a hiccup maybe.
Of course, Steam going down like that, would be a major event, especially if no support for customers was provided. There would be a huge backlash, and not just against Steam, it would be against that model of game provision in the industry overall. So DEVs and especially PUBs would want to be seen to be doing the right thing.