SDL exists because of Windows 8, 8.1, 10 and any subsequent rebrand of the software. The DirectDraw functionality which the Software Renderer used to be reliant upon is deprecated (meaning "abandoned; if we break it by accident, we'll remove it before we fix it") - not changing SRB2 to not rely upon it would mean its days were numbered, any future version of Windows able to kill it with a single accidential patch. As it is, performance is already in the shitter in the DirectDraw build on modern versions of Windows.
I forget exactly why the sound library was changed, but I assume it's because SDL is a single library capable of handling both drawing and sound, versus having two libraries linked at once. Maybe efficiency. I dunno - we lost a few things from that I'd rather have around, but the benefits of SDL as a whole outweigh the loss of different music speeds and sample rates.
also there's a chance 2.2 might ship without a directdraw version, so get in your bug reports about things that don't work nicely before then