-
Doesn't a chain naturally stretch over its normal lifespan (before it's worn out)?
They don't stretch, at all if you're talking about the plate stretching over time.
You're talking about chain wear mainly cause by the pin getting smaller as illustrated, which give the impression the chain stretch;
TBH, it's not something people should concern with as long they check the chain wear/bring it to a reputable LBS regularly.
Replace the chain before it wear out to that point then (they don't stretch in the literal sense).