Do they really last forever?
NO…But they do last a lot longer then timing belt.
But if they do break, they do not destroy an engine like a timing belt?
Having a belt or chain has NOTHING to do with if it will destroy the engine if it breaks or slips. It has to do with - if the engine is an interference engine. My first vehicle with a timing belt was my 72 Vega. When the belt snapped…I just pulled over…put a new belt on and was on my way. My Dad owned a 71 Toyota Corona…when the chain slipped - it destroyed the engine (interference engine).
Chains don’t usually break…they slip. If a chain slips a couple of teeth in an interference engine - it’s the same as if a belt breaks - destroys valves or the engine.
While chains last a lot longer then belts - if you drive long enough the chain will need to be replaced. Usually over 250k miles. I have over 260k miles on my 4runner and so far there’s no sign of chain rattle. The cost of a chain replacement (before it breaks) is 4-5 times the cost of a belt. So in the LONG run a belt might be cheaper. Much cheaper if you can do the belt yourself. Many backyard mechanics can replace a belt…but don’t possess the skills to replace a chain. While the principle of both is the same…the chain is a lot more complicated because of the introduction of lubricant. Now you have to deal with Gaskets (usually oil pans)…etc. Belts are a dry system.