Plastic can be plenty rigid, especially carbon fiber variants. As used on high end bicycle and exotic car frames. When designed well, also more resistant to dings, and lighter than aluminum and titanium bodies and frames. As well as thinner structural components than metals (Apple seems to obsess about every thousands of an inch). Main downside other than cost is thermal conductivity but that can be easily be remedied with a small square of aluminum or fan (or maybe their new vapor cooling system). Using plastics means simpler antenna issues, no cutouts needed to allow transmission and reception.
As to the added expense, I'm already on record to be willing to pay $1,500 for a premium product under 2 pounds.
Edit: maybe I'm lowballing. Just looked up my original Apple Store purchase of my 2016 12" MacBook 1.3Ghz and 512GB of storage at $1,684 plus another $183 for AppleCare. I wanted the 2015 12" MacBook when it came out but the processor was a bit gimped. I wisely decided to wait for the 2016 but it was hard. Zero issues with the 2016 but it is end of life because no one is updating software for the last available MOS 10.13.6. Most likely I will have to buy something next year so I'm hoping this does materialize. When the M1 MacBook came out, I bought one but returned it in days. Who knew there was such a big practical difference between 2 pounds and 2.7 pounds! I pick up my laptop and move it dozens of times daily and I hated that heavy MacBook Air.