1) As already noted above, today's young college students lean conservative. Besides, registering people to vote and getting them to show up at the polls are two different things. RTV was good at the former, but hapless at the latter.
2) Back in 1992, the average MTV viewer was a college student. But in the intervening years, MTV has moved further and further away from music and into cheap tawdry pop culture pablum, and the average age of its viewers has plummeted. It no longer matters how much support MTV throws to RTV now, because 15-year-olds can't vote.
You are wrong that they are no longer a danger. In fact, RTV is more dangerous now than before, for the very reason you cite: because their target audience is younger.
The Left knows that its positions fail the test of logic and reason. Instead, they promulgate their agenda by promoting irrational emotion using disinformation. This tactic works best on "blank-slate" minds, hence they target children, as well as the under-educated (which is why schools are deliberately dumbing-down). The earlier they can start their indoctrination, the more deeply rooted that indoctrination becomes. Brainwashing is easy if you have virtually unlimited access to a target mind over a long period of time. That is why the Leftists control both the schools and the media.
Today's "insignificant" 15-year-old, is tomorrow's voter. And by the age of 18, most children will have spent more time with (and watching/listening to) Leftists, than they will have spent with their parents.