This is an Android feature thats pretty useful. Example: You listen to music on VLC and open a Video in your browser. The active Media playback is dominant and the older one gets paused. Optional: when the active media playback is manually paused again, the old media starts playing again. The setting for that could be manually set, for example to avoid ads disturbing music? If your browser mutes media automatically, volume should be evaluated too, so that only hearable media can replace the previous one.
There are cases where this sometimes already happens; for example when you have KDE Connect set up and your phone receives a call, all playing media known to the Media Player widget is paused, and when the call ends, it's played again. I'm a bit skeptical about the value of pausing currently-playing media when a new stream appears, though. I worry that this would feel too magical and annoy people, and they wouldn't figure out how to turn it off. And it would surely need a bunch of heuristics to avoid triggering at the wrong times. You definitely wouldn't want your music player or video to get paused because your chat client sent a notification that played a sound, for example. Or for your music player to pause when you navigate to a page in the browser that displays an ad with audio.
So yeah, I think we won't move in this direction, sorry!