Let me put it another way

If Schleicher were to put a service limit of 20 years on a glider and just write it in the manual would you say that is the new limit or does it require more?
Would you still fly it if it was older than 20 years?
Would your inspector continue to inspect it on condition?

These questions are more out of curiosity than anything else.
Maybe the manufacturers could do this to us one day. Who knows?