Normal people don't give a crap about which decoders to enable to make their favourite soap opera to play. Some people barely grasp the concept of decoders. And you want a GUI on top of it all? Be my guest...

The decoders-support package takes a negligible amount of space and its sole purpose is to be a meta-package codecs enabler. Look at the source to see how it works. Install one package = have all the codecs people want in one place. Simple.

If you want you are free to go to the terminal, find out which GStreamer packages support which codecs, install them, use the commandline tracker mimetype registering tool to manually add support for containers and other miscellaneous file types and restart the multimedia framework. Then build a GUI on top of it all. Or you can live with a decoders-support package and forget about all this.