The mechanics that made the holy trinity work are mostly long gone (threat management). Right now, the holy trinity is a vestigial appendage that only serves to make it more difficult for people to get into the content. It also restricts design decisions by making bosses hit so hard that you need tanks and healers, preventing creative solutions to encounters.
This doesn't mean we shouldn't have classes with tank and healer toolkits, but those roles should be altered to fit a model that does not require them.