I think you are confusing "what is historical" with "what is technically better".
Again, you will probably lose the adapter weight argument if compared against plastic. Not only is plastic less dense than aluminum, but a separate plastic piece can have internal voids in it where material isn't even needed. Little bit harder to do that with a solid aluminum body, so the volume isn't even the same. I can literally calculate the equivalent weight if you want. Actually, wait, I don't even need to calculate it. I just need to find my scale, which has recently gone missing...
OK, everybody remember this little fella?
It represents, fairly realistically, the aluminum material for the chamber and threading as it would sit, implemented, in a ULE body.
Here's roughly the same functional piece implemented in PLA:
Even a solid SLS nylon version still wins: