Chainsaws are high energy producing internal combustion machines, usually ran under load, cutting wood. How long they last depends on how hard they are ran, the fuel/oil mix, and the quality of the materials used to build them. I have several chainsaws built in the late 1960’s to early 70’s that run great and have seen serious usage, and in fact, are still in use. I suspect the quality of the cylinders, pistons, connecting rods, and bearings is very good. They also have never seen ethanol used in them, it deteriorates seals and invites corrosion, the fuel to oil mix has been carefully followed, and they are kept clean, sharp, and in good operating condition. There are lots of newer Poulan, Craftsman, and other saws in the shed that can be used, but it would require some tinkering to get them going, so why bother? As far as buying a new, modern, quality saw, keeping the right fuel (again, I admit a bias against any ethanol blends) and oil, maintaining it correctly, and running it from time to time to keep everything lubed up, it may make it 50 years, too…but I wouldn’t bet on it. Plastic just doesn’t age as well as metal, in my opinion. In other words, I think other answers suggesting twenty years is realistic, but optimistic for today’s saws.