Why is signing former soccer superstars vital for the MLS' existence?

During the last decade the MLS has been investing strongly on improving their academies in order to upgrade the league. Since back in 2008 it was imposted the Homegrown Player Rule that allow teams to sign players from their own academies, clubs have regularly have young standouts. But still signing world soccer superstars seems vital for the league, why is it?

It happens that the European leagues, much richer than the MLS, have got enormous incomes from television. According to CNBC the MLS current television deal provides around $90 million as revenue, while each and everyone of the five principle European leagues have deals that generates more than 1 billion euros per season. This money is then distributed between the teams.

For this reason, up to now the main source of revenue from a commercial perspective for the MLS are the massive commercial contracts signed by their superstars. This means that the incomes of, for instance, Inter Miami, are much bigger from the commercials that Gonzalo Higuaín film using the Inter's t-shirt than from the TV directly. This phenomenon started with Los Angeles Galaxy signing David Beckham back in 2007, and remains until now. 

On the other hand, MLS teams has lately discovered a new way of creating incomes for themselves: selling players to European leagues, which can pay substantially more than teams from the rest of the world. For instance, the deal of Alphonso Davies to Bayern Munich has provided Toronto FC of 13.5 million dollars, which still can grow up to 20 million. This is a lot more money than what the Canadian side make from the TV. Non-the-less, still the mayor income for MLS teams is to sign former soccer superstars.