This is something I’m very interested in too. RootsFinder is an excellent tool for working together collaboratively but at the end of the day there is only one owner and the collaborator has less power. I have separately asked that a person with edit access should be able to export GEDCOM of the tree. That as a minimum ensures that a collaborator can obtain their own copy of a tree in the event of the death of the owner. Obviously transferring ownership is only useful while the owner is alive, so having something in the line of ‘beneficiary’ is an alternative. A simple option when trees are already shared may be to have more than one owner for the tree, or perhaps a secondary owner who can claim full owner rights at some time when needed.
It’s obviously a bit complex but the simple aim in my mind is to be able to pass ownership from one person to another either in life or afterwards.
A workaround is to have a tree owned by an independent user account that can be ‘owned’ by more than one person but that starts to involve more subscriptions etc etc. We have done that for some trees that are managed by a family history committee where it is worthwhile, but not so practical for individuals.