If people do not vote they lose any input into the result.
Representative democracy doesn't work like that. Just because I abstained from voting, or voted for some other fellow, it doesn't mean I am no longer entitled to any representation in Parliament. I may not have the Government I wanted, but I still have an elected MP to whom I can take my concerns.
Democracy is when the people who are allowed to vote actually use that vote, you simply cannot count the people who did not bother voting as 'being for the status quo' - if you did we would never have a change of government.
A periodic vote for a representative is very different to a permanent constitutional change, for the reasons I gave earlier. I agree that a requirement for a "supermajority" may be a difficult pill to swallow, but to make such a major and permanent constitutional change on a simple 51% majority of an indeterminate number of the population seems every bit as inadequate.
In a representative democracy, my personal view is that referenda are a wholly unsatisfactory way to conduct Government. They are nothing more than a snapshot of public opinion, there should need to be a clear mandate for such major and far reaching changes. You could end up with a 51% majority based on public mood that one day, influenced by such things as the weather, bogus newspaper headlines that morning, or some other transient issue that day. Even if referenda are only legally an advisory to Parliament, which most are, the damage they do to democracy is immense - Government is consumed, public mood is soured for years, and the influence of those with extreme views is inflated.
But, as they say, it's all water under the bridge now.