Duel made a great point; M:TG won't go out because it has such a stable audience.
The pro-tours, etc. are just testaments to how popular it is, Hasbro would be making a big mistake to drop it. There is not a doubt that it's extremely profitable, think about it; 15 pieces of cardboard doesn't cost too much, and even employment doesn't cost that much when it comes to such a huge product. There is no doubt that they're making crazy money, and the only thing that could bring Magic down is if Hasbro got greedy and tried to make more money, turning a lot of players away from the game.
It's something to not worry about, at least for a couple years, as there are already plans and money into future expansions.
S'all,
Dustin