To survive, a DAO has to attract the participation of significant active stakeholders to the stream of building and voting proposals for the evolution of the specific underlying subject (dapps, networks, etc).
The underlying subject, in a living and vibrant DAO, inevitably evolves. Why then, shouldn't the relations between a possible Parent DAO and the Sub Dao change? Such relations should be mutable and dynamically constructed on the basis of an interest on the part of a DAO to align itself, on the basis of principles, underlying subject or other, with other DAOs. This alignment could be long lasting, temporary or even ephemeral (i.e. on the base of a single proposal).