Optional voting is a feature, not a bug. It ensures that the people who care most wield the most influence. If they collectively take the rest of the (apathetic) population astray, more people will start to care and swing the country in a different direction. There's a term for this kind of self-correcting system, but it escapes me right now.
Regardless, changing to mandatory voting would do absolutely nothing to solve voter apathy. It could theoretically have the opposite effect, as now the people who care deeply have less influence because their votes are watered down by people who only vote to avoid a fine.