Dota's always been this toxic. It's just that the game's changed so drastically so often, even compared to other mobas, that a lot of old-timers (me included to some degree, but especially people who played a lot back in WC3) are feeling pretty bad about how the game plays now. Especially when they have heroes they're attached to that get drastic reworks or the flow of the game changes so much that you stop enjoying it.

It's not really about the game even getting better or worse in a vacuum, it's about just not being the game you loved in the first place. 2018 Dota 2 is an extremely different beast than 2012 Dota 2, which was about when I started playing. I can definitely see Sing getting bored with the team-heavy roaming and rubber banding when he was so used to going around solo ganking people with Tiny, Lina, Mirana, Kunkka, etc. Those kind of strategies just aren't as strong anymore and haven't really been for years, ignoring the brief Tusk meta.