Can anyone explain me like I'm five, why this major transition from PulseAudio to PipeWire is happening during the last year?
There are some things PipeWire makes a lot easier to do. For example, I have an electric piano that I can connect to my computer and play. I had to install a special software called "jack", because PulseAudio would have me wait before I could hear a note I played. Jack was not simple to use, but even if you were good at it, there were problems. Imagine you wanted to play along with some music. Well, bad luck, some so…
You absolutely can, PA is then present in JACK (and JACK in PA) as a single sink/source that you can route like any other app - so not as flexible as PipeWire, but not as problematic as you paint it either.