In Australia, citizens have a duty to pay taxes, defend their country (if required to do so), serve on a jury (if their name is drawn), and vote. That's the cost of living in a democracy.
If voting was compulsory, only lunatics would vote. I wouldn't voluntarily pay taxes, sign up to fight a war, or serve on a jury either - I'm not crazy; but I think it's important that these things get done.
Hm, thinking about it, maybe I was wrong. Maybe you can actually express "I don't care" vote in the mandatory voting system as well, by voting for a randomly selected candidate (and these contribution will average out to same votes for all candidates). But it's rather impractical compared to simple non-mandatory voting.
That's a technical issue. There are voting systems with points, and you can vote against someone rather than just for them. And you can vote for many people. I've said it once and I'll say it again, FPTP is a tyrant's idea of democracy.
One argument is to stop efforts to disfranchise voters. If voting is mandatory things like photo ID law in US won't fly (or at least would be much less effective). It also affect who politicians pander to. These days common strategy is to pander to radicals because it's easier to get them to vote. That would disappear if everybody has to vote.
When "none of the above" wins, it usually means new elections where all old candidates are banned or some other procedure to resolve complete distrust to current politicians.
But then the original point stands. Such a system doesn't fully allow you to express legitimate feeling that you don't know. If majority expresses "I don't know", it doesn't suddenly mean "I don't trust old guard" vote. That's misinterpretation of the voting result.
Also, you can express "I don't trust old guard" in the non-mandatory voting system as well. Just vote for the new candidate. So it's actually simpler system than yours and more expressive.