The right to vote implicitly implies a counter-right to not vote. Having the right to abstain or register a protest in the direction of society by not participating in elections is as important as any societal good that may come through compulsory voting. Introducing compulsory voting does not always have the added effect of compelling supposedly apathetic or disenfranchised people to participate but instead can simply lead to a rise in the number of ballots that are spoilt or left blank. Similarly, there are vulnerable groups, such as the elderly or the disabled, who may not physically be able to vote. There should at the very least be an opt out for vulnerable groups on health grounds.
The societal good of mandatory voting and its ability to increase the turnout of elections and their democratic legitimacy far outweighs any inconvenience non-voters may experience. Even if apathetic voters elect to spoil their ballots or to leave them blank they will have registered their opposition, which is progress compared to the current electoral system where such votes are not formally counted.
[P1] Having a 'right to vote' insinuates there is a counterpart 'right not to vote'. [P2] People should have to right to opt out of having to vote.
Rejecting the premises
[Rejecting P2] The societal benefit of all people voting counterbalances any concept of a 'right not to vote'.