I've had lots of bikes over the years but the one that lasted the longest was my Peugeot UO-8 Mixte. Got it used in 1978.
It was my city/commuter bike until last year when my boy started taking it to school. Last week he gave up on it complaining of a strange shimmy. So I took it for a ride and looked for the shimmy. The shimmy was very prominent.
Turns out that the downtube was cracked 3/4 of the way around. The crack started under the old shifter boss and promptly disappeared under the cable clip (used with thumb shifters). I'm tempted to fix it, as its a very loyal bike. But there are visible cracks along the brake bridge and minor one's elsewhere.
Sometimes you just got to let go.
Amir Avitzur
Ramat-Gan, Israel