The actual original MTV is still on the air; these were music channels launched not long ago mostly in Europe.

It's been quite a ride for MTV, although the journey is closer to an end than ever. But the pioneering music video network that started in the U.S. got almost no carriage on cable there when it first signed on, and faced a possible short existence. 

And then a marketing agency created one sentence spoken by major music figures that changed everything. And it all started with an inspiration from a breakfast cereal for kids. 

How “I Want My MTV” Saved the Network From an Early Grave

I think Terry O'Reilly highlighted this on an episode of CBC Radio's  "Under the Influence." It's a great story about the power of a catchy tag line.