This. Absolutely this. Being a healer in Vanilla was a thankless task. You were completely crippled outside of a group, you couldn't farm, you couldn't grind and you'd even struggle with gathering because if you had to kill anything at all to pick up a node someone else would swoop in and steal it before you could kill the mob. At best you might have a set of blue DPS gear you'd managed to scrounge up from 5 man dungeons, but without the talents to really support it your ability to do anything alone was seriously limited.
My guild got through more healers than any other role by far. Not because they were getting poached but because they usually rerolled to a DPS. Maintaining enough healers for a 40 man raiding team was a task in and of itself. If you're the biggest and best and guild on the server you might have healers lining up to join, everyone below that is probably struggling to keep enough healers just on the roster.
Things might be different come Classic of course, but I'd expect not, the core issues are going to remain. Skilled and geared Healers, of any class, are always going to be in demand simply because of how many you need for a raid and how rare, relatively speaking, they'll be compared to DPS. Things might be different for players who find themselves in Hardcore guilds who are looking to get everything down ASAP, but those are always a minority of players - And one most of us here probably aren't going to find ourselves in.