Well I am certainly not an expert but heres my thoughts.
As bitcoin grows, alts will move up with it, maybe at a slower rate than bitcoin, but they will move up with bitcoin. There are many amazing projects out there that are very promising and so even though bitcoin is the leader, once people learn more about this space they will buy into alts that have good tech. Bitcoin got the ball rolling, its going to be a bright future for cryptocurrencies. Another thing to keep in mind is that most alt coins will actually die out eventually. many of them have been started up in ICO's which gained them a lot of money, but if there is no solid tech behind their projects they are doomed. So yes many alt coins will disappear along the way but the few that are solid projects will survive.