It's reasonable to conclude "if this is a potential problem, and it's easy to solve, we should implement that solution assuming the solution does not cause other problems." But negative karma was deliberately implemented to keep disruptive individuals from continually posting after being heavily downvoted. To my knowledge, while there are people who've created sockpuppet accounts, the issue of someone creating a sockpuppet to get around negative karma has not yet come up (and if it has, the people doing it managed to create a sufficiently clean break from their earlier records that their taking up new handles was probably for the best.) On the other hand, people who've been regularly disruptive and had their posting throttled by negative karma is an issue that has come up before.