Eh, back before dual spec people played healers so I don’t know if it’s true nobody would play it if that’s their only role. I don’t remember many people respeccing constantly back in vanilla. Maybe the sweats in the top guilds did but I’m not sure if anyone was doing that back then in my guild. So if you played a priest, druid, paladin or shaman back in vanilla, probably 80% of them were healing and not playing other roles.
But it definitely is true that far fewer people want to tank or heal. As evidenced by the fact that whenever I queue for ZG, it’s 90% dps in the queue. Can’t say I blame people, I find healing extremely boring. The most fun I’ve had healing was mage in SoD. I know blizz redesigned some of the healing classes to have similar gameplay in later expacs and that’s the kind of gameplay I’d enjoy most as healer, where my damage passively heals targets/groups.