What type of things do other games do in this situation?
Some people mentioned team games or team maps, so is this a social problem? If you only played 1v1 would you still want rank decay? Or is this about letting your team down? If the change allows mid-rank players to achieve a win rate >50% then it just creates a smurf-lite system.
In 1v1 just eat the losses.
In team games, one bad lane can lose the game. Maybe this is what makes gap so popular since it's less prone to this. There's also thermo, which is quite literally one lane. Other games aren't so dependant on one person at the casual level, and the one's that are also tend to be very salty. Maybe there could be some push towards maps with lane sharing for casuals. Maybe 8v8 winter dual was always the best map after all.
Anyway, only solution I can think of is to play easier maps.