This isn't true.

Swing voters exist. Moderates exist. Single-issue voters exist. Occasional voters exist. These are observable facts about the world.

The four groups exist in large enough numbers that they decide elections. Die-hard party loyalists exist, committed non-voters who'll never ever vote exist, but they're fixed quantities and are practically irrelevant.

I agree with the statement that what really matters is whether you can convince someone to vote differently - but, yes, of course you can! Trump has run three times and only won twice. Obviously there's something that can convince people not to vote for Donald Trump, because it has already happened.