While it would seem to be a better beast, the material involved and the people cranking attachments down may cause concern for ripping through the material. It is the only thing I can think of. Otherwise, your concept seems to make loads of sense.
Because the tapered screwhead would be centering on the MLOK accessory and not the rail. Remember that the screw hole is in the backing nut that is flat against the inside face of the rail which is also flat. You would just be centering the backing nut to the hole on the accessory which is a relative position and of no benefit. The locating feature of MLOK are the lugs that should be spaced the same as the slots. The KMOD has the tapered nut against the tapered inside of the rail which would center it to the rail. Hope that makes sense in words because I can’t draw the pictures that are in my head.
The rail doesn’t center the accessory in MLOK. The clearance between the recoil lugs on the backside of the accessory and MLOK slot has play. The screw holes in the accessory have play in between the holes and screw.
There are MLOK accessories on the market that use tapered head screws, just not many.