I see your point. 58% is a kind of low turnout, but still is more than 50%, so... I think +50% would be considered enough quorum to decide something. Not ideal, but fair enough.
And I give you a question back:
Why do you think the 42% of legal votes didn't vote? This could be the reasons:
- They didn't like any of the 2 mayor party candidates.
- They felt that partisanship took the monopoly of making decisions, so no ideas from independent candidates could be implemented in reality. So there's no point to vote.
- They just don't care about politics.
How can giving them an option to dissapprove a candidate would make them eager to vote? Third party candidates could really have a chance. I have learned while seeing the last 3 US elections, that some voters have an affiliation with a third party candidate at some degree, but as elections come closer they swing to republican or democrat just to avoid their opposite candidate to win. And that takes its toll. If you vote for a candidate to counter the other, and your choice turns to be worse in the long run, probably you won't vote again at all.
Negative Voting could solve this decision dilemma.