Hate to put a damper on this thread (I thought my 708 was my forever bike) but I’ve now heard of a second 708 frame cracking. https://forum.cyclinguk.org/viewtopic.php?t=163522
In my experience, that’s unusual for Reynolds frames.
Mine went on the down tube and chainstay. The other bike on the top tube.
So I’d say, enjoy your bikes but check them regularly. That said, don’t worry too much. They are steel and well engineered so failures are unlikely to be catastrophic.