Fleet Footwork nerf will do nothing to remove its dominance on marksmen.
Current nerf on PBE :
bonus move speed : 30 % 15 - 30 % depending on level.
This will not change the fact you'll take that rune to have a safer laning phase, and with marksmen itemization costing more, you'll want even more safety to reach your spike.
This rune is un-healthy, rework it. There's grasp for safe laning phase, but you'll have to trade DAMAGE for this. When FF disappear, marksmen still have the option to go Grasp of the Undying.
Safety has to have a cost.