Youtube has been giving in more and more to pressure from copyright lobbyists recently. A huge amount of youtube content is now blocked where I live (Germany) because of copyright issues. This is just more of the same.
While it might seem bad, actually it is a good thing. A single company having the lions share of any particular online media is really really bad. Even if it is a relatively good and customer friendly company like google. Their monopoly on video clips has made them a target for the copyright cartels. What the copyright cartels never seem to grasp however is that their behaviour is actually beneficial to consumers. Very soon after google blocked 90% of their music videos in Germany, a new site opened hosting all that content. Having more options about where to go has many benefits. The most obvious benefit is that more sites are harder for the copyright cartels to police and shut down, especially when they are hosted by shady companies in former eastern bloc countries and other net neutrality safe havens where the cartels can't touch them. Secondly google is unlikely to stay so friendly for ever as we are seeing. Even if they want to be, they are often subject to external pressures and forced into behaviour that is not really in anyone's best interests. If google had been able to maintain their monopoly on online video there may have been bad consequences in the future, such as secret censorship and propaganda programmes. It is better this way.
Google (yes they still have other monopolies) 'youtube to mp3 service substitute', and if you don't find anything try again in a month, and then a month after that. The demand is there, it will pop up quietly somewhere, and all the users will quietly move there. To be honest I didn't even know youtube had an mp3 service as there are so many easier ways to get your mp3's anyway. I use bittorrent, and if I want something from youtube I use clipgrab, and if I just want the audio from a video clip I use mediacoder.