As you or may not agree, 10k is a very low value for the number of songs that a user is able to save into your music. Although you may consider that the majority of users do not save that much songs into their libraries, the truth is that there are a LOT of music fans that LOVE to have their libraries organized and very complete. The whole idea of paying for a Premium account is not to have this type of limitations.
However, I believe that the majority of the fan base understands the technical limitations of having such a huge amount of connections on your database. That said, with technology such as Redis and Cassandra you're definitely able to do it. Additionally, you've a lot of developers inhouse to take care of this and you also already had the time to improve this limitation. You're not a startup and you should be able to deal with this.
That said, I really hope that very soon this limitation is being removed or, at least, upgraded.