A compelling case for automatic enrolment, making voting easier (on a holiday, electronically, by post, from jail, etc...) everything that would alleviate voter suppression...

But without the above, you're punishing exactly the people you were intending to help, those who can't afford to get to the polls and / or aren't able to register.

With all of the above, it's difficult to see what the extra step to making it mandatory would really achieve.

You're not making things simpler by policing something that currently isn't.

There will be many appeals, legitimate cases of people getting caught at work, having accidents, delays in the post or just being so busy they honestly loose track of the days. It's removing, not adding to, that extra level of complication and enforcement that makes the case for a flat tax rate. Even if you're merciless, you have to track people down and take them to court.

Filling out a census, or tax return, paying taxes, they are all infringements on ones liberty (and doubtless cost a fortune to administer) but it's a payoff for a better functioning society (in theory). I don't think you've made that case for the mandatory vote here.

Sorry if I'm being harsh, you're getting the brunt of many arguments I've had on this subject over the years. I always vote, and it infuriates me when others don't - I've even made the journey just to spoil my ballot in the past, when there's no one worth voting for. But democracy is about choice, and non-participation is a valid one, in my opinion.