In a best case scenario yes. If the system goes down, or they lost the rights for it, or go bankrupt, or any one of a number of scenario's that have happened in the past occured, then you're out of luck.
Well actually, that's not always the case.
As long as your account is tied to having acknowledged purchasing the game prior to its removal, you're free to redownload it after deleting it even after it's been removed from the marketplace. Same even applies with Steam. I got OutRun 2006: Coast 2 Coast a while back on Steam. Sega's license with the Ferrari at the time expired, forcing them to pull the series from all downloadable sources. They regained the license, but OR2006 is no longer on Steam to purchase. I can still very easily delete the game and redownload it without any hassles, simply because the game is tied to my account.
The ONLY one you have to really worry about is the Wii, but that can be fixed as long as you register the console before any possible break down occurs. In that sort of situation, a call to Nintendo can have them transfer your account over to a new Wii, should you have to replace your old on.