You seem to look at Aros as some kind of reserve only interesting if MorphOS and AOS fail. They will both not "fail" or it depends how you define fail. They have both their supporters who swear on it, they are both depending on old and in case of AOS expensive underpowered hardware and this will not change. So they will stagnate and slowly shrink because they are not able to win new users.
I do not see Aros as "reserve" because it is the only one that supports modern hardware (different platforms) and new concepts.