It sounds like you are unaware of all the fail-safes incorporated into the Model 3. This is a very well engineered car. I'm not saying it's impossible for it to catch on fire but age will not add much fire risk.
Salty water in the electrical connections? Even if you drove it into the bay it wouldn't catch on fire. It's engineered to be protected from that. Chaffing wire would likely be detected and shut off. Even so, it's designed with minimal combustibles around the wiring. Time will tell but I don't expect to see a bunch of Model 3 fires just because the car is 10 or 15 years old.
I have not heard a single person claim the Model 3 needs no maintenance for half a million miles. Everyone recognizes it needs tires balanced and rotated, coolant changed, brake fluid replaced, suspension components replaced, etc. You are likely referring to people who claim it needs a lot less maintenance than any ICE car. And that is absolutely true. The 500,000 mile life of the drivetrain has yet to be proven but there is not much evidence that it won't go 500,000 miles. So, yeah, I think it's likely in most cases.