MRAM is not an SSD replacement
These emerging NVMe storage class memory technologies (i.e. RERAM, MRAM, PCM, et al) are an entirely new class of storage (though some of the technologies have been in research for many years). They are generally 1000 times faster than SSD and have characteristics closer to DRAM than SSD. In the future, when coupled with SCM, it is likely that SSD will become the equivalent of a spinning magnetic hard drive.