Players on the wing often move about 10 mtrs in board before play on is called, they get away with it on part because the man on the mark goes with them. That will have to stop if the man on the mark cant move, so players will be stuck on the boundary line, so will be more likely to kick down the line, increasing congestion.Agreed, technically if a player goes off the mark, including walking or "wandering" back to kick for goal, it should be play-on. These rule changes have all the hallmarks of an arms race - a tit-for-tat response that will see coaches invent new ways to limit the intentions of these rules.
I also expect players will stop manning the mark straight away- staying close enough to stop playing on, but not too close they are stuck there, until their team is sorted.