youtube and vevo are much more popular than MTV ever was, plus the exposure is worldwide.... In some cases this is a way for artists to present their art in a different way, some love this exposure still....
"MTV" like it was in the 80s and most of the 90s will never be back, because like you said it's already there on the web...but at the same time it's highly likely that an actual MTV as a new network would get better cable TV ratings than a 47th channel devoted to 24/7 low budget reality tv shows. When MTV/VH1 first went to exclusively reality TV programming there were only one or two other cable channels doing it, MTV basically started the whole trend themselves. Now there are dozens of reality channels, I guess the difference is MTV is a reality channel specifically going for young people. I still find it hard to believe with hundreds of garbage channels that there's no place for a music video channel, I mean some of these cable networks have to have tiny viewerships yet they exist.
When I travel abroad it seems most other countries have a music video channel. When we were in Greece and Spain somewhat recently we put on some their music video channels pretty much any time we were in our hotel rooms and the same channel would be playing at most bars and coffee shops we went into. It's not like they don't all have youtube as well.