The bigger a company is the faster it burns money. A slip up in revenues because of a flopped product or competition and all of a sudden you are in a liquidity crisis. It is just as easy for a big one to go bust as for a small grocery shop. All it takes is a lawyer-CEO or Gabe Newell run over by a tram...
But there is a larger issue here. I could not care less about Sierra going bust - my Half-Life 1, Opposing Force and Blue Shift are on CDs and all I need to install them is a CD-key. For HL2 etc, which is Steam-based it's not enough to have a copy of the game. You always have to ask daddy for permission and when the daddy's gone so is your HL2.
This is the price you pay when you trade physical possession for DRMs.
Regards