

If a single issue is enough to make people put their vote into, why wouldn’t a larger party simply make that promise themselves, as long as it still somewhat aligns with their party goals, or is not in the way of their goals, and eat the single-issue platform’s pie?
And if anything, we already have single-issue politics, right within our FPTP system, and I don’t see why this wouldn’t happen under ranked ballots or instant runoff as well. Instead of forming a party, they lobby, and whichever party adopts their stance will win their votes, along with whoever else they can influence. There already is a subset of the electorate that are geared into thinking that way.
I don’t think there’s a good solution to mitigating single-issue politics, perhaps other than good education about our governments and institutions. It certainly isn’t solved under PR, and, as you said, could possibly lead to the proliferation of small single issue parties (though I believe larger parties will absorb their vote by promising the same while offering more), and it certainly exists even today under FPTP, just not as a party but a lobby group, and it will no doubt exist similarly as a lobby group in other winner-takes-all system.
To the “brainwashing our kids” crowd, my stance is that the kids can decide for themselves if it’s brainwashing, as long as we aren’t actually doing that, and is instead simply equipping them with the ability to think on their own. So I’m not bothered by them, and I think we should make that narrative clear enough, with experts in and out of power to have their say, and the rest can complain all they want. I do understand that that doesn’t always work well in our political climate; just look at the carbon tax, but if we hold ourselves back just because some crowd might fight back, and essentially do nothing, based on the trajectory where things are going, I fear that we’re only sleepwalking ourselves into ruin. This applies to adopting PR as well.
In other words, I’d rather we say that we’ve tried to do things that we have good reasons to believe are good and may actually steer us in the right direction, than go for something that might please more people but is no different from our current trajectory.