Why not just have the application set the sink directly using the audio-sink or video-sink property?

Yes, it's possible and I know setting sinks by properties is very convenient. However, please think about a strange circumstance that we can't modify  the upper layer player codes and I don't know if the upper layer uses audio-sink, and video-sink properties. In this case, I used this approach to provide proper sinks only when the upper layer doesn't specify certain sinks.

What about ./configure --with-default-audiosink=xyz ?

I am very appreciate for your attention even though I missed something important to explain.
Perhaps, this patch would be only for me, but I want to help if there's someone who have similar situation. :)

This is my application layers.

--------
Player (Uncontrollable)
--------
Gstreamer Helper Functions (Controllable)
--------
Gstreamer (Controllable)
-------- 

In the player layer, they call the helper function like this, but I can't access and modify the codes of the player. To help understand, I'll describe pseudo codes.

  pipeline = create_pipeline_by_helper ();

I provide the helper function, create_pipeline_by_helper, as a library. It looks like that.

create_pipeline_by_helpder() {
  
  playbin = create_playbin();
  
  conf = get_configuration_from_file();

  set_configuration (playbin, conf);

  return playbin;
}


The configuration file has several properties related to playbin or pipeline like this.

[playbin]
#default-video-sink=debugvideosink
default-video-sink=releasevideosink
#default-audio-sink=debugaudiosink
default-audio-sink=releaseaudiosink


The requirement of my patch was from this idea.
For convenience of debugging, I just want to replace default sink element without compilation and with the closed upper layers.

I think you should just set it in the application using the properties.

(I don't like these default defines anyway.)