Part of the problem is that fixing Newton is more than just adjusting the equation; it requires making up a whole new constant, which to some is worse than making up a whole new class of matter. Constants don't have explanations; they're just like saying, "I don't know why it works out this way, it just does." It's better to have a reason that explains why something happens than to throw up your hands and say "let's just add a new constant to make things work out."