I get a lot of questions from people about this very topic so here are a few good things to know.
The average laser tube is only rated for a shelf life of two years. I've seen many units that are significantly older than that and still functioning and there's a very good reason for that.
Laser tubes hate air. What we call a co2 laser isn't necessarily just carbon dioxide. Depending on your manufacturer and model there could be other stabilizers in there to help increase the lifespan of the tube and the air not one of them. Oxygen slowly seeps in through microscopic gaps in parts of the laser tube which diminishes its ability to produce a laser. Even if it's perfectly sealed the base operation of how laser works changes the chemistry of the laser tube over its lifespan which is what essentially kills it.
Don't buy an extra tube. Even sitting on the shelf these things are taking down and there are plenty of American and Chinese suppliers they can provide you with a new laser tube fairly quickly.