Those are two real effects, but together they wouldn't compensate for the rate cuts.
About 8% of the population is uninsured. The uninsured population skews younger, with less healthcare utilization (Medicare already covers everyone 65 and older).
Another comment in this thread estimated billing overhead at 8.5%. Medicare for All would eliminate some, but not all of this, since Medicare is still a claims-based system. You would remove a lot of overhead around prior auths, which I agree is a good thing, but could be achieved with more focused legislation.