Schedule 40 or schedule 80 don't matter, you can BS yourself all you want. If that pipe loses any of it's original elasticity, which I assure you it does as time goes by, it becomes brittle and can't keep up with the pressure changes in the system. What people don't understand is that it isn't the max pressure that always kills the pipe, it is the pressure changes. The pressure in the pipe changes every time you use the blow gun or the grinder. Each of those pressure cycles are stress on the pipe. When the accumulated stress is too much, the pipe fails. It will happen to steel or copper too if it is in use long enough. That is how pressure vessels are tested for long term reliability. Water heaters get thousands of 5psi cycles, which replicates turning a faucet on and off. Eventually the vessel will fail at a contaminated spot in the steel, next to the weld if it is good, or separate the weld if it is poor.
