After your first post, i've thought about your points, and i've changed my opinions a little.
I agree, we should replace the default lemmy UI as a whole. Out with it. But photon wasn't designed to be a "default" UI. It was designed as an alternative.
The point of a default UI is to be as accessible as possible, yet still functional. That's why most users on the fediverse usually use a 3rd party client (Including me, i use photon + raccoon on mobile). While the current lemmy UI checks the "performance" box, the functionality box? not so much. It feels like everytime i use it i am overwhelmed with information density, and the auto expand feature implementation is just bad. Photon does the opposite. It checks the functionality box, but scratches the performance box. Even on a modern PC, photon is choppy for me, and the frosted glass effect is not helping.
I don't think we should have a default lemmy UI, as in, every instance uses the same UI. As i said in the first post, it should be up to the instance maintainer to choose their own UI. This comes with its own problems. Lemmy.world's photon is outdated, and so is lemdro.id.
Ultimately, this is in the hands of the instance maintainers, and Xylight himself. He is rewriting photon for more performance, so my answer may change.
Good, but slow