I found at least a partial answer here: Frappe CRM Live Launch & Release - #4 by shariquerik
So, CRM will not only be integrated with ERPNext, it will replace the CRM module currently available there. According to the same person, a migration path will be provided.
At least in the longer term, this settles the question how ERPNext and CRM are meant to coexist.
In my view, this makes the question of licensing even more pressing. Because this move basically means that ERPNext users, who wish to keep using the newest version, will be forced to accept the AGPL going forward.
As mentioned, I believe this could lead to an exodus of certain corporate users, or the formation of forks, internal and external. Which IMO is not good for the long term success of ERPNext.
If there is a need for really new, purely independent apps, then license them AGPL for all I care. But something that is so closely related to the functionality in ERPNext should be on the same license IMO.