Steel mountain bikes are nothing new. Throughout the 1980s, nearly all mountain bikes were made from steel.
This changed in the 1990s, as aluminium became more popular due to its lower mass. And in the current mountain bike market, almost all frames are either aluminium or carbon with very few steel options.
The truth is that steel lasts. A long time. If it is appropriately rust treated, a steel frame could easily last for the entire lifetime of its owner And become a hand-me-down bike.