Welcome, Emvale!
I have MS and can't answer your specific question but
it sounds as if you've been unnecessarily scared by one of those doctors with a "crystal ball." Doctors shouldn't make predictions about how long someone will live, IMO.They really can't know.
I found this about MMN (last updated in July 2022):
Multifocal Motor Neuropathy - StatPearls - NCBI Bookshelf
Here at Neurotalk you will find discussions of MMN in the Peripheral Neuropathy, Neuromuscular, and Autoimmune Diseases sections. You might find it useful to enter "multifocal motor neuropathy" in the search window if you haven't already done that.