The last time I picked up a G70, which I believe is made of cast iron, at least judging by the weight, the very first thing that came to mind was "Boy, I would hate to lift this damned keyboard up 6 times a day (3 jobs). And, when I owned a G-800 I really didn't think it sounded nearly as good as my PSR-5700, which had a plastic body and weighed nearly as much. Consequently, the G800 was sold to an old friend that loved Roland arranger keyboards, while I lugged the PSR-5700 around for another couple years. The PSR-5700 was way ahead of its time, but when the PSR-2000 became available at about half the weight, I quickly purchased two of them and sold the 5700.
From my perspective, as a retired, full-time, musician/entertainer/singer, that steel case really had no bearing on my performances. Additionally, if I recall correctly, the interior of that case was not painted or protected at all. Therefore, if you have ever played seaside outdoor jobs, especially at night, that steel case would quickly become a rust bucket.
If you google plastics stronger than steel, you will find there are many of them available and at reasonable prices. The world's largest sailboat uses 5 carbon fiber masts that do not require any additional support, unlike the heavy gauge aluminum masts that must be supported or they will bend and break. The carbon fiber masts weigh 5 times less than the same size aluminum mast.
All the best,
Gary