I'd have to ask why have a separate assembly that was purely for vanity (whether Apple's interest in promoting their product, or the user's pride of ownership)? Every part/assembly comes at a cost, and each one is a potential point of failure, both in initial factory quality control as well as during ongoing use, repairs, etc. Imagine the complaint posts, "Why should I pay for/carry the weight of something that has no functional purpose?"
The old glowing Apple was brilliant because of its simplicity - there was all that display backlight, why not steal a bit to illuminate a translucent plastic panel? It even had its uses for troubleshooting - if there's evidence the Mac booted (the LEDs on the Shift Lock key or MagSafe connector) but there was no light pouring through that little window... potential fluorescent backlight failure.
I haven't researched this, but one potential reason for the phase-out would've been a change in the design of the display panels - even before a switch to micro-LED, all it would've taken is a module built with a solid metal back (which is the way iPhone displays have been built for as far back as I can remember). Now, the iPhone display modules probably need a bit of electromagnetic shielding, as the CPU is right behind the display. It also reduces the chance of a shattered display puncturing the battery or cutting a ribbon cable. Laptop lids don't have those particular concerns.
But as much fun as the glowing logo often was (the Apple advertisement in countless movies and TV shows where Apple loaned the filmmakers a laptop/the personal announcement of "I've got a Mac"), it could be annoying in a darkened meeting room.
The highly-polished Apple on the backs of iPads and Macs is a bit more understated, but as long as the light hits it from the right angle, it's still a very noticeable Apple advertisement.