Yes there will always be borderline offsides, but by using feet only it will be much simpler to implement, with fewer errors. If a player's foot is offside by 1 mm and the goal stands, or 1 mm onside and it's ruled out, who cares? Those would be rare instances, and it'll even itself out over the short run. If a player is that close to being offside/onside both the attacker and defender can't complain if it the decision goes against them.
Also someone mentioned that a player can score with other parts of his body. Again, who cares? The idea is to have a rule that stops long punts and goal hangers, and that wouldn't change using feet only.
As they say, K.I.S.S.