Sorry to be so blunt, but nothing lasts forever.

Frames flex in use, and eventually will fail. When companies offer lifetime guaranties they're just playing the odds.

Add to that, heavy riders and steep terrain mean higher stresses and flex, therefore faster failure.

Chainstays are especially prone to failure from hard climbing. Changing chain tension, flexes the rear triangle to the right, flexing the right stay into an S bend. Steep hills usually mean the rider is using a small chainring, giving him greater leverage to tension the chain more. A heavy rider climbing steep hills with a 24t granny is the triple whammy, and it's only a matter of time.

Of course bikes could be built stronger, but we're in a sport where weight is a premium. They don't build sports cars the same way Ford builds F-100 pickups, likewise bikes are built to a purpose.

The bright side to all this is that companies do offer those lifetime frame warranties, and most or all don't have rider weight exclusions. So heavier riders with below average frame life are covered.

The other bright side is that chainstay failures are very unlikely to cause injury (though I'd keep bus or cab fare with me at all times).