Shared employment on a 50/50 split requires twice as many qualified crew, and anyone employing yachties know crew can pick and choose the jobs their offered.
Rotating crew accumulate experience, sea time and qualifications much slower than full time people.
Rotation comes with salary sacrifice in most cases. Employers need to pay extra to cover logistics, insurance and not all owners are ready for that. Therefore rotations not offered on all boats, and not for all positions.
For rotation to be sustainable there must be;
2.Overlap between rotations for handover
3.Crew work more than their off, even if that's off the boat in a supporting role
4.Contingencies for those times when rotational crew leave or get injured
5.Clear operational standards, with checks and balances
6.Proper logs and records
8.Revalidation and further education plans
Rotation is wonderful if you can make it work, but it quickly becomes a nightmare when people fail in their duties, suddenly leave and employers see zero benefit.
At the end of the day rotation must be mutually beneficial for employees and employers.