During their end of year blowout sales last year I started buying heavily from Steam.  I've bought from them in the past, but I've bought far more this year due to them drawing attention to the frequent (ridiculously good) deals on great games old and new.  Prior to this I would have categorized myself as one of the people who liked to have the physical copy for the assurance of ownership it brings.  
 
I've changed PCs once since first using Steam, so I know it's very simple to get your games to a new PC (DL steam, login, DL games).  My only concern then is that one day Steam shuts down.  Now how do I get the games I've bought?

they have said they will let you download all of your games and let you play them without steam, they've tested taking the DRM out and it has worked, so I wouldn't worry about it.

They will have a patch or something where you can continue to play the games and stuff regardless of them closing down. You won't get to buy anymore games, still I think they will release a patch that would enable you to continue to keep playing.

It won't shut down -- it's far too popular. If it ever did I'm sure they would figure someway to let you keep your games without the DRM steam provides.

But what about way, way ahead in the future. Like 20 years? People still have their old NES' kicking around and sometimes pop games into them, or Super NES is getting up there in age too. Will things like Steam, the PS Store, XBLA last that long? Will we still have convenient, instant access to our games? I'm talking about in comparison to having a console and a hard copy of the game.
  
If we don't have access to our games, they will just be pirated on the uber-emulators that will be created in the future. EDIT: Even if we do have access, people will still emulate anyway. Look at how popular SNES and NES roms are.

Technically Valve would have no authority to remove DRM from titles not published by them sold over Steam, so if Steam goes down, those games will probably be lost to you forever.  Same if MS shuts down 360's version of Live and doesn't retain back compat, or Sony shuts down PS3's PSN, or Nintendo Wiiware/DSiware...
 
This sort of DRM perma-lock has happened with fairly large non-gaming projects before.  It will happen to some online gaming platform at some point.  It's one reason I still want to stick to physical disks.

" But what about way, way ahead in the future. Like 20 years? People still have their old NES' kicking around and sometimes pop games into them, or Super NES is getting up there in age too. Will things like Steam, the PS Store, XBLA last that long? Will we still have convenient, instant access to our games? I'm talking about in comparison to having a console and a hard copy of the game.  If we don't have access to our games, they will just be pirated on the uber-emulators that will be created in the future. EDIT: Even if we do have access, people will still emulate anyway. Look at how popular SNES and NES roms are. "

I never use them, so I don't know.

Someone would buy the rights out from under them and take command.

Over the last 4-5 hard drive upgrades/changes ive never redownloaded any of my steam games. Just transfered them to the new PC. If you keep backups of everything you can keep playing all of the games in offline mode. Keeping an old hard drive with the games n whatnot on is no different from keeping old CDs/Cartradiges etc.  Some people argue that they could lose the backup, it gets corrupted and other such nonesense. But thats no different than an old game disc getting snapped or damaged in some other way that its unplayable.
 
When it comes to ubisoft style PC DRM though, youre gonna have to do some shady business to keep playing those games if they close up. 

I cannot count the number of times this question has been asked on the Steam forums. In fact, I think you should create the same thread over there just to get a reaction out of the regulars. Always a fun way to pass the time! :P

" @LordAndrew: There's a lot of Steam games that retain their own DRM.  But besides that, unless Valve has some special agreement that allows them to remove Valve's own DRM from other publishers' games (which I highly doubt), they wouldn't even be able to remove that DRM.  Publishers / developers allow Valve to publish with a guarantee that their product will be protected.  Valve can't legally do whatever they want with it, anyways. "