I'd have 2 example:
GW1: once people figured out how to sync retarded builds together in random arena they litterally drove away all the legit players, nowadays all u find is bots and sync and maxi 20 players rather than thousands.
SWTOR: people can already sign as group of 4 on 8v8 and everytime there is a sync of 4 they stomp the randoms who stop signup until they can play again.
But then again since all meaningful stats such as k/d ratio arent recorded anymore on a leaderboard such as yellow gremlin, aoc pvp is 100% casual.
Group sign-up in random queue is only a logical followup since all hardcore players already left.