It's not a huge flaw, since our current voting system (in any country you care to name) allows exactly that, and your scenario is not observed in the wild, since it is illegal.
> your scenario is not observed in the wild, since it is illegal.
I don't mean to be snide, but I've witnessed a great deal of illegal things, so "since it is illegal" is not that convincing.
Even if you discount the threat of violence because of the possibility of police intervention (e.g. you report being threatened), consider the notion of vote buying: a rich person just says they'll pay you lots of money to vote for them. This isn't necessarily something people would want to report, given the possibility of financial benefit, especially if the person is down on their luck.
At any rate, if electoral fraud is something that is "not observed in the wild", people sure do make a big fuss over it (see http://en.wikipedia.org/wiki/Electoral_fraud for some references).
This is actually why Helios voting (which is end-to-end auditable) doesn't prevent coercion either: your voting is done 'at range' (online), so a coercer can just stand over your shoulder. Just about the only surefire way to prevent coercion is to somehow physically give someone security while voting.
Okay, well, anyway, my point still stands; even if coercion is possible in the current-day low-tech system, if we're going to discuss a cryptographic replacement, we might as well restrict ourselves to the baseline "best-studied" ones so far, virtually all of which include coercion-defeating mechanisms.
At a minimum, if a physical presence were the norm with cryptographic voting and there was an option to be absent (like today), then we would be in an identical scenario with regard to coercion while gaining the end-to-end auditability of the cryptographic scheme. It seems like a net positive, although many critics of cryptographic voting see the increase in complexity as unworkable.
You're missing the fact that it's very possible to use coercion to discourage certain groups from voting at all. Just target the groups that are likely to vote for your opponent, and you've essentially used coercion to manipulate the election.
Because people might ultimately think "well my vote probably wouldn't have mattered" when the line is 8 hours long on a Tuesday, but they will definitely care when they get fined $30 - but they're not going to simply argue the fine should go away, because it will be a little reminder of "oh yeah, and you didn't get a say in your government either!"
Australia has mandatory voting as do other countries. We don't have problems with it.