"First of all, I think that is true, if you are a musician, particularly on the come, that you do have to end up in one of these musical centers, some way, to be viable, saleable and so on"

John Sebastian is recommending that if you are an up-and-coming artist, you need to be based in one of the significant music centers in order to be successful. He thinks that in order to be practical and saleable, you require to be in a place where there is a big music industry presence. This might indicate relocating to a city like Los Angeles, Nashville, or New York, where there are many chances for musicians to network and find success. Sebastian is implying that if you wish to make it as an artist, you require to be in among these cities in order to be taken seriously and have a chance at success. He is also suggesting that if you are not in among these cities, it will be much more difficult to make it as an artist.