So when you said a miner is dead, this means that probably ( with high probability ) one or more sha256 chips are KO. Is this chip the most fragile component on a miner?
There are other components that fail, but most of the time is a dead chip or a few of them, if you think that is a small problem which can be easily and cheaply fixed then you need to do more research, the average fix for a chip would be in the $100 range including labor, of course it depends on where you live, but if you are going to buy a used gear for $400 today, chances are by the time it needs fixing you can buy it for $200, fixing a single chip for $100 or even $50 does not make any sense, so more often than not and especially with the old gears, once a single chip dies, the whole board is rendered dead as far as logic is concerned.
Another common issue would be the PSU, many gears are known to have fragile PSU, and the problem is, most gears today don't use a universal 12v PSU which you can buy of anywhere, they rather use their own designed built-in PSU which in most cases are hard to find, many people have perfectly working miners that need PSU and can't find them.
and overclocking them reduce drastically their lifespan ?
I am not sure about "drastically" but at least in theory overclocking any gear will reduce its lifespan, but it's always a question of how solid that piece of gear is, I have seen overclocked gears that run over 100c all day long for months and months without an issue, on the other hand, many gears that run on stock settings died way too soon, of course, if the latter group was overclocked - chances are they would have died a lot sooner, but it all boils down to how solid that gear is.