Sure, they exist, but they aren't worth wasting any time or campaign effort on - unless you're suggesting that the campaign should simply promise these people what they want to hear without any intent to actually deliver.
Pandering is really the only way to bring such people into your camp - and it's the tactic that the GOP uses to pull in fringe groups and religious nuts. I don't think the Democratic party should start operating that way, even if it means losing elections.
My opinion is not that single-issue voters should be disenfranchised. Everyone should vote.
My opinion is that no reasonable person should waste their time listening to the opinions of single-issue voters or trying to have any kind of political discussion with them, and that no serious political campaign should waste time trying to pander to them.
Side note, this "you":
If you can’t grasp nuance, you really aren’t qualified to have political opinions.
wasn't directed at you personally, but broadly/generally. I thought that was obvious in context, but I was wrong. My apologies for the confusion.