Voting shouldn't be mandatory, because voting is ultimately about choice. Requiring people to vote ultimately denies them of the choice of not voting. Right now, a majority of eligible voters sit out any given election. Can we really expect who are too apathetic to vote on their own initiative to make intelligent and informed decisions for the rest of us?
Maybe so. I suspect that people who are involuntarily voting for the first time might be strongly motivated to support candidates who promise to repeal mandatory voting. In a three way race between a Democrat and a Republican who support mandatory voting, and a Libertarian who opposes it, then the Libertarian could win with a clear majority of involuntary votes.
So Libertarians oppose mandatory voting on moral grounds, while Democrats and Republicans would likely do the math and oppose it on political grounds. The only scenario that I can imagine mandatory voting coming to pass is if the President and Congress see the country collapsing after the next election, and they want Libertarians to get elected to office and absorb the blame.