To me, the only reason why you would want to mandate voting is if you want to increase civic participation (or, more cynically, you are a political party who has done some research and you have realized that such a law benefits you more than the opponent). I think a law like this would not make people engage, but make it look like they are engaged. Because of this, I think it is pointless, and if it is punitive, then it fails to accomplish what it sets out to do and just punishes people for no reason.
I don’t like superficial policy. I want policy to actually attempt to fix problems rather than try and mask them. This doesn’t fix issues like people being unable to vote due to work, or people feeling abandoned by politicians and not wanting to give them a modicum of support, or people just feeling crushed by the system itself and seeing no point in it all. This doesn’t even attempt to look at root causes.
This doesn’t address the inability for many people to run for office, be it because they can’t afford the money needed to get started, or because they can’t afford to live off the politician paycheck for one reason or another, much less afford to take time off work to campaign.
I also think that not voting is fundamentally a vote. Sometimes the two choices are just so abhorrent that you can’t bring yourself to vote, and is that not a valid political stance? Is it not an intentional political choice? Isn’t that what voting is in the first place?
Sure, you could have a system that lets you vote “nobody”, but if that’s allowed, then why are you mandating voting anyways? This subverts the point of that law, and it means the effective use of the law is to punish people who vote for no one in the wrong way. What is the benefit of a blank ballot or a “nobody” ballot over no ballot?