My first thought echoed The Vulture, thinking prog rock would like the visual medium. On the other hand, they'd need to cut down their epic songs into three and a half minute single versions to get airplay.
MTV play was responsible for breaking several obscure bands in the 80s with frequent play of their videos. One that comes to mind is A Flock of Seagulls with I Ran. Part of the reason was that not all bands made videos then, so MTV played whatever they had. If MTV was established earlier, there'd be a lot more domestic fare, and American bands would probably dominate the market more than they did.
As to whether MTV 1976 would have the same impact as MTV 1982, let's ask how many homes even had cable television then. According to the infallible Wikipedia, it wasn't until 1972 that there was original cable programming; cable television had originally been used to transmit free TV in areas where terrain made traditional transmission too difficult. (My family got cable in 1981, because we moved to such an area.) NCTA has the following figures:
1962, 0.85m subscribers
End of 1970s, 16m households
End of 1980s, 53m households
Not much to go on, but MTV would clearly reach fewer people.
Another issue is MTV's original refusal to show videos by black artists. That might have been feasible with the music of 1982, but in the late 70s, you couldn't show videos by the most popular disco performers with that policy.