It is not the keybed that adds to the weight on the G70..It is the metal chasis...
As an example when I updated the MediaStation with the lower metal chasis...empty it weighed about 20 pounds..the same in plastic would be about 3 pounds..tops
I would rather have a heavier metal built keyboard that I know will hold up..rather than the cheap plastic construction..
Roland did use Aluminum construction in the D70 days, and it was lighter than the G70 by 15 pounds...
I would think aluminum would be durable and well constructed..
BTW: My favorite all time keybeds are the beds I own..The MediaStation and the G70, are still the best semi weighted keybeds..
I am spoiled, and can never be happy with the PSR, Juno and PA beds..The PA900 is an okay keybed as are the Tyros and PAx..but not as playable as the MS and G70...in my opinion