Most single-engine aircraft were already restricted to 61 knot stall speed (and I think all MOSAIC-eligible ones were).
I’m a big proponent of MOSAIC (as I was of BasicMed), even though the MOSAIC rule change won’t directly help me.
The fact that the stall speed limitation closely matches the existing limits for light singles shows where the FAA draws the safety lines. Although MOSAIC apparently allows twins, they keep the stall speed limitation at a sane number.
The only thing I don’t fully understand is them differentiating between 59 kts for a sport pilot, and 61 kts for a light sport aircraft. It feels a little arbitrary to draw those lines differently.
The NPRM document explains it somewhere around page 200. It’s important to note that it’s 59 knots CAS clean vs 61 knots CAS dirty.
The argument seems to be that a sport pilot is operating with less training and less oversight and so if something goes wrong and they can’t get the aircraft fully configured for landing they’ll still be able to operate and touch down at 59 vs whatever the clean stall speed would be for an aircraft with a dirty stall speed of 61 knots. That margin could be fairly large (10 knots or more) and the difference in energy between 59 and 71 knots is massive.