The concentration of media ownership is a problem and it needs to be dealt with accordingly. Regulation, breaking up monopolies, more public media, socialization, all these are parts of the solution that needs to be done. We need to counteract the natural tendencies in a capitalist economy toward concentration now.
No, concentration of media ownership is not a problem, because new journalists can enter the market if there is enough money to be made. A concentration of media ownership can allow for efficiency and can allow the media to reach a great deal of people. As long as the market is open to allow for new participants, concentrated media is not problematic, because there is still room for competition.