Yes, the ethics complaints were a problem, but at least finish her first term. That's what Reagan and Bush would have done under the circumstances.
Quitting, at this point, wasn't the answer.
Sarah made it clear why she was leaving early. To stay in office, after deciding not to run for re-election would only embolden those who had been filing complaints since Aug. of 2008. She didn't think it right to waste the taxpayers' money fighting those complaints, because any use of her or her staff's time would have done just that. If she had to spend half her time fighting complaints, she didn't use that time profitably for the state.
Given those same circumstances, I think Reagan and Bush would have done EXACTLY the same thing.