You argument works in favor of Spotify too. If they sign on 100 top artistes, it will devalue the portfolio of music held by the top 3 - for reasons you explai

You argument works in favor of Spotify too. If they sign on 100 top artistes, it will devalue the portfolio of music held by the top 3 - for reasons you explained. Thus, the top 3 are incentivized to collaborate with Spotify.