Why would it be a thing? Rifle receivers still have picatinny rails on the top of the receiver where the rear sight would go, and most M-lok and KeyMod handguards still have picatinny rails at the twelve o'clock position where the front sight would go. For those handguards that do have M-lok all the way around, it is not at the same height as the rails on the receiver, so it would not be possible to mount irons. Picatinny alternatives like M-lok, KeyMod, SquareDrop, etc. never were really meant for mounting sights that have to maintain zero.