Perhaps I can answer that. I don't view plastic as cheap or automatically a "toy". The Tyros is a serious keyboard and tool for many. They will sell becasue of the features and sound set. Not its ability to withstand a spilled drink or tip over. A Flight case can ore than protect any keyboard.
I also view it as a way to keep the price down for a keyboard that is not universally used as a touring unit. Steel is heavy and for Arranger keyboards in particular not necessary. I think most would agree as long as it sounds great and functions having a light keyboard to carry around is a bonus.
(See Bose Lt1, LOL)not to open a can of worms.
Yamaha Tyros 4
Yamaha Motif XS8
Martin DC Aura
Breedlove ATlas Solo
Bose MOD II PA