Our experience is the same as others, replaced for the first time after 14 years even though it looked fine from above. When removed it was clear that the rubber has cracked at least half way through around at least one third of the circumference. I will not leave it so long next time, even if disregarding the insurance issue.
BTW the “hard” carbon deposits in the exhaust elbow are not carbon at all but sooty blister corrosion of the cast iron (which is why it is hard). Investment cast stainless elbows exist and permanently solve the problem.
The membership association for owners of Dufour Yachts