From what I remember and Kevo can fill in the gaps ... Mike (Micheal Stokes) the original coder of Shareaza had his HDD crash on his development laptop ... amongst the lost code was the MediaPlayer ... he had recovered the rest from other backups. This was as Kevo explained during the time Shareaza was closed sourced. He never did get around to re-writing that set of lost code, but re-used the already compiled .dll file because it was the least buggy component of the Shareaza equation ... hence it was low in the priority.
As for including the new instead of the old, well it is the right logical path once the old one has exhausted all it's use, it is time to change things. 64bit compilation, compiling with
DEP mode, removal of last Closed Source component, and more. If you insist in having the MediaPlayer of old it can be done ( I think so long as Plugin API hasn't changed). But it can only be available for 32bit version for the reasons already explained.