It's true; if you introduce a 10 cm or 20 cm or - if you're Arsene Wenger - a 1 metre margin of error, or whatever, you just move the edge and will still very tight calls at a different point that will be debated.
However, I think it would be much more palatable for fans having that debate to say "well, it could have gone either way, but he is clearly [x] cm ahead of the defender, I can accept it's probably offside" (as if any fan ever talks like that).
What we saw yesterday is a prime of example of what's not palatable.