There's a very clear and simple reason the population is decliing

For me, I’d play more often even after the weekly challenge has been completed if it wasn’t for the fact that outside of challenges, there isn’t much there beyond it. Thankfully, when forge comes out, I could spend hours just messing around and making my own maps, even if they end up being more about set pieces or trying to make rollercoasts or little play areas to mess around with. Plus the custom games scene would just explode.