It is not unusual for chimney sweeps to take literally buckets of nesting material from these chimneys. This nesting can catch fire directly, or it can block critical air passageways between layers of metal chimney pipe, allowing the chimney to overheat. Both scenarios routinely cause housefires. Most after market chimney covers do not correctly address the problem, and can often make the situations worse. A careful screening of all potential nesting areas with the proper sized screening may be in order.
Models go out of production and manufacturers go out of business.
A factory-built unit will reach the end of is useful life when repair of the unit is no longer possible, particularly if the components that are necessary to maintain the listing are no longer available. Keep a close eye on an aging unit, and be prepared to send ole faithful to the great recycling plant in the sky before she fails completely.