Lord You Know Who 
Doesn't it seem like this DID happen, only in the future depicted in G5?
Considering Fluttershy would be dead of old age, and that G5 Ponies seem a LOT less tolerant than G4 Ponies, they would most likely stab Discord with a shard of Chrysalis Throne Rock first chance they get. Or at least, throw him in a Chrysalis Throne Rock-covered dungeon to rot.
I highly doubt the G5 Ponies wouldn't give Discord his much-needed comeuppance.
Considering Fluttershy would be dead of old age, and that G5 Ponies seem a LOT less tolerant than G4 Ponies, they would most likely stab Discord with a shard of Chrysalis Throne Rock first chance they get. Or at least, throw him in a Chrysalis Throne Rock-covered dungeon to rot.
I highly doubt the G5 Ponies wouldn't give Discord his much-needed comeuppance.