As mentioned on previous occasions, these couplings are doing something they're not ideally suited for.

They're predominately used in uni-directional loading at steady speeds. In the Morgan they're subjected to load reversals and varying load conditions.

Stop/start driving and hard acceleration/deceleration will shorten the life compared to gentle driving on longer trips.

Running the engine in a narrow rev band may well help, but the attraction of a big V Twin is surely that it can pull from almost nothing.