Thank you all for these answers and advices.
The electrician tested for resistance the cables from engine batteries to starter motor.
The cables were fine (couple of ohms as expected) but the positive switch tested at 1300 Ohm and the negative switch at 80 Ohm.
Completely out of whack and I am ordering them with Amel.
We also discovered that we have a loose contact at the stater motor, that we were able to resolve only after we unmounted the starter motor - access is difficult - on the Yanmar. We rethreaded the starter motor screw.
Finally we replaced the starter relay which was not working consistently and did some electrical housekeeping on the starter circuit.
Why the switch failed?
Possibly the loose contact
Possibly the fact that I never turn them on/off - as someone mentionned it in the thread - great insight.
Measuring the resistance was new to me and I can do it with my own meter. It is easy and I got a crash course :-)
Thanks again for your help
Papeete, French Polynesia