I expect it's for "build quality" not weight-savings. By gluing and riveting everything, nothing moves so the frame remains more rigid and less chance of stuff working loose inside.
Only buying a Mac once and upgrading it forever is anathema to Apple. Heck, it is anathema to any manufacturer.
Honestly it probably costs Apple more to do these "replace rather than fix" designs. I mean there is a reason they kept tinkering with the butterfly-mechanism and it probably was because the costs of replacing the entire upper-case every time one failed was getting out of hand and having a material impact on the bottom line of the family. Apple charges what, $750 for a repair? Even if the part and labor costs Apple $250, that is probably half of the margin on a base laptop. Replace it again and they're negative on the whole thing.
Considering how many people complain about Apple not updating Macs every six months when a new CPU or GPU model is released, one would think most users do replace their machines on a very regular basis.