And no, fumbles are not "inherently bad" and unfix-able by nature. I did it. I made them good.
We need to stick to our design principles. You know how much I like a well-thought-out houserule. So, if people have come up with houserules for fumbling attack rolls, there must be a reason they did it. What drew them to it? The first question when you are modifying the existing rules is...
What's the Problem?
The usual logic goes: if critical successes are a thing, why not critical failures? If you can have a 5% chance to whoop some ass, why not an equal chance to fail? Doesn't that kind of chaos add a little bit of crazy fun? Simple enough, right?