If it wasn't for professional marketing teams, who are some singers who would tank as a professional singer?

That's saying that it ONLY takes a "professional marketing team" to make a star. Marketing teams can only be successful at their jobs if there is something TO market. Whether a singer has actual talent, or just good looks and riveting stage presence, or just a good gimmick to exploit, that is still a "product" to try to sell to the public. There are some singers that have talent and have successful careers (that can be defined as being able to make a living from singing), but they may not be "famous"...that is they may have strong local followings, or they are well-known and respected among their peers, but not household names. Even so, there are plenty of examples of singers that had all the marketing money can buy, but whose singing careers simply did not take off. For example, Lisa Marie Presley was the daughter of Elvis Presley and was MARRIED TO MICHAEL JACKSON.

Success is a combination of many things, including sheer luck to be at the right place at the right time, and plain hard work and sacrifice from the singers themselves.

Bieber