I guess with ARM they can either design their own cores, or buy arm's, but not other arm architecture licence holders. With riscv they can do their own design or choose from a larger list of competing vendors
At most, they'd get some licensees to comply for a single generation of chips. The next one would have given them enough time to rid of ARM.
For most licensees, it wouldn't even be possible to release one generation like that, as most are licensing IP blocks from multiple vendors, which might internally use whatever ISAs. Obviously, the owners of these IP blocks would hurry up and move to RISC-V, too, in order to protect their business models.