This review at PC clearly, no pun intended, states that the clear front and side windows are plastic.

the r13/14 cases are short lived and not designed to support motherboard upgrades.  their acrylic window is only good looking when new and would not stay scratch free for long.

Forgot to mention that an all plastic outer case is terrible for thermals. Metal and glass dissipate heat - plastic traps it in the case. I guess I didn't know that about Alienware. Rather disheartening.

. . . just wait till you look on the inside

lol, meaning...?

proprietary motherboard, proprietary PSU, down-clocked RAM, throttled processor, insufficient CPU thermal solution, nice graphics card, nice CPU (when not throttled)

Thats not true. I'm a CNC Engineer and create plastics with a high temp tolerance, and my side panel is standard, with a metal shield inside, xD and the front panel is for inside flow, so i think Dell Technologies did a great job.

We talk about temperatures who are not affect any kind of material, glass or plastic.Its a ethic option from brand, i gues.

The front clear plastic is solid, and it impedes airflow. Then there's more plastic behind that plastic, impeding it even more...

I don't think so, impedes from what? you can drill a plastic product as smooth as you wish even at a tolerance 0,005, those plastics we have on a alienware have simply nothing to do with the airflow or temperature. And beside of that...glass can break fast

Per Dell, the clear side panel is made of a special plastic that acts as an emi filter. The solid side panels have metal shielding.  Glass does not provide the emi filtering.  Evidentually, Dell feels the need for the emi filtering.

impeded air flow vs. unimpeded air flow (click photo to embiggen)

No, it's a shell to hide the very industrial chassis underneath.

It's there simply for aesthetics and looks, and it does impede air flow because it's a second shell around the case.