I recently reached the 10,000 song limit in my library, which is a great disappointment. As a loyal customer for over 3 years, I have thoroughly enjoyed your product, so much that I have shifted to using it as my main source of music collection. There must be some way that you can enable "super users" to continue to file and organize their music over this arbitrary limit.
There may not be many of us, but (and this should be obvious) those of us that actually reach this limit are your core constituents. We are passionate about music and your product, and I can tell you that I have gotten at least 20 people to sign up to Spotify Premium over the past couple of years as I have raved about it. I will continue to do so, with the caveat that Spotify just isn't yet ready to become a full-fledged music library of record for musical aficianados, much to my disappointment.
One more thought: You may find interest in the creation of some type of super user account that would allow you to charge SLIGHTLY more for those of us who desire or require a higher song limit (say, unlimited...) without having to alter the accounts of your millions of users. Bottom line is that unlimited should mean unlimited, especially in this age of big data, but a 10,000 song cap with no alternative is absurd.