What is to prevent the OSS community from making a more restrictive DRM standard based on ogg vorbis with some DRM-ish layer? Does this mean that the only legal streaming format will then be ogg-DRM-vorbis?

The RIAA and the other middlemen must really be worried that they are going to be cut out of the equation when the artists realise that they don't need to give up 99% of the revenues and could just as easily hire an online company to distribute their works for them at a much lower cost. Legislating a certain format for the online distribution of music would turn the tables again and force the artists to deal with another middleman, in this case the company that owns the rights to that DRM format. The RIAA could simply buy those rights to that particular DRM and they would be guarenteed a revenue stream for quite a few years into the future.