ALSA is incapable of software mixing, the dmix plugin handles this. So PA isn't redundant in this capacity.
As for OSS, no one uses it directly (or at least shouldn't). Many programs will use the OSS emulation offered by ALSA (which doesn't support software mixing). The interesting thing about OSS is that it was originally open source, became closed, and became open source again with version 4. Although, now we have shifted to ALSA, it will probably never be as popular.