Can you vote for only one candidate? Certainly, if you want. This means you fail to rank the other candidates relative to each other, simply saying that you like that one choice more than everyone else, and dislike all the rest the same. Your choice.
But, if, in the end, your one choice doesn’t have enough support to win, you’ve missed your chance to have a say in selecting the best-alternative among the other candidates. So in this case, for you, the point of the preferential ballot is lost. We can lead a horse to water…
If everyone voted like this, the whole election would transpire exactly as-if it was an FPTP election, and, while we’d get no benefit from the preferential ballot in that case, we would be no-worse-off than had we simply stayed with the FPTP system. If there happens to be a FPTP majority winner, it doesn’t make a difference, either way.
But the point of the preferential ballot is for those many cases where we don’t get a majority FPTP winner. While some voters might indeed persist in the old ways, despite a better way laid before them, some, hopefully most, won’t, which will accordingly allow us to move beyond unsatisfying, unrepresentative, plurality results.