Why attract the signal when you can attract the noise?
Mass media has been with us for a long time though. What's different about it now? Are people focusing on fewer media sources as time passes, or more media sources? If we're focusing on more media sources, wouldn't that play well in your idea that dumb votes will cancel out?
This changed in the 90s with the 24 hour news cycle, which shifted the news from a duty you performed as a citizen, to entertainment for hours a day. Then social media meant you could argue about it all day long -- again, for entertainment. People have always been susceptible to media manipulation, but they've never spent as much time willingly exposing themselves to it and reinforcing it on each other.
I don't believe the number of media sources is an issue. The truth is readily accessible. It's hard to be genuinely informed on a matter involving expertise, but that just leads to the conventional dumb votes. Now, people are seeking out misinformation, which is different from ordinary ignorance. It used to be hard to target people for actively disinforming them, but techniques for that have been improving for decades.
The only reason people are concerned now is because Trump manipulated the electorate in a different way than politicians usually do, and this is a great violation of tradition.
It's true that this ratcheted up to 11 in the last major election, and was co-opted by somebody outside of the establishment. In the end, the establishment pivoted, and are getting nearly all of their goals met. More, in fact, than under their own plans. So they're pretty happy about it, and see it more as an innovation for their benefit than as a violation.
I don't mean to be picking sides here. In theory, the same tools are available to their opponents, and I'm sure they're trying to. In practice, I suspect that they will not fare as well using the same techniques. They'll continue to develop others, just as they had used social media innovatively in the past two.
Democracy exists because voters are too stupid to form their own decisions. It relies on the hope that stupid decisions are largely random, while informed decisions will all tend in the same direction. It takes only a small minority of people to know what they're doing, if the uninformed ones can be counted on to mostly cancel each other out.
That's obviously a fraught assumption, but mass media puts an increasingly large wrinkle in it. It takes only a small thumb on the scale of uninformed votes to overpower a minority of informed voters, and new techniques increasingly enable people with a lot of money to apply that thumb. If only 1% of voters are informed, and a misinformation campaign can sway 2% of otherwise evenly-divided voters, it becomes the dominant factor -- even if the informed voters are unswayed.