It seems to me that it's easier than ever for someone to broadcast "niche" opinions and have them influence people, and actually having niche opinions is more acceptable than ever before.
The problem you should worry about is a growing lack of ideological coherence across the population, not the elites shaping mass preferences.
And that certainly means niches can flourish, the dream of the 90s.
But I think mass broadcasting is still available, if you can pay for it - troll armies, bots, ads etc. It's just much much harder to recognize and regulate.
(Why that matters to me I guess) Here in the UK with a first past the post electoral system, ideological coherence isn't necessary to turn niche opinion into state power - we're now looking at 25 percent being a winning vote share for a far-right party.
With enough data, a wonky-enough voting system, and poor enforcement of any kind of laws protecting the democratic process - this might be a very very small number of people.
Then the discord really is a problem, because you've ended up with government by a resented minority.
The content itself (whether niche or otherwise) is not that important for understanding the effectiveness. It's more about the volume of it, which is a function of compute resources of the actor.
I hope this problem continues to receive more visibility and hopefully some attention from policymakers who have done nothing about it. It's been over 5 years since we've discovered that multiple state actors have been doing this (first human run troll farms, mostly outsourced, and more recently LLMs).