There is virtually no upper limit on the service life of a properly maintained outboard motor. Premature deaths can always be explained by lack of preventive maintenance, failure to address problems as they arise, or just plain neglect.
If the routine maintenance is done, there is rarely a need for non-routine maintenance.
Sitting around idle for long periods is harder on a motor than regular use - rubber parts dry out and crack and rot, gasoline turns to varnish, internal engine parts lose their coating of oil and corrosion sets in, etc.
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"The metric system never really caught on in the states. Unless you count the increasing popularity of the nine millimeter bullet." -- d. barry