First, traditionally, that doesn't make any sense in RPGs. Especially Warlocks are one of the classic "squishy" glasscannons. Second, that is not addressing the problem at all. Even if you made every class able to tank with a unique spec, you won't have enough tanks. It's an attitude problem of the players and that is the one thing Blizzard cannot change, despite all their carrots and attempts to do so. Some people don't like the responsibility of 2 out of 25 making or breaking the fight for everyone. Some don't like the boredom tanking brings with you. And some simply prefer to play something more competitive, where you can match up yourself against others, so they play dps. And others are the guardian angels, who are happy to sit back and save every one else's life. They're the healers and pull their weight in any raid. It's not a setup problem, it's not a class problem. If anything, it's a design problem.
Blizzard has tried to address this by turning tanks into dps specs pretty much. That royallly pissed off both tanks (that prefered to be the defensive masterminds that they are) and dps (who are now threatened on the dps meter) as well as healers here and there when tanks now opt to go for offensive stats rather than defensive stats. And really, nobody but the top echolon of guilds knows how to employ tanks properly these days. So that whole idea was a huge dump.
Parallel to that they turned to boss design for a couple of years, making it more interesting. Well, kind of. Instead of just standing at the boss spamming whatever, because honestly, it doesn't matter except for the few crucial one shots that you have to time your CDs for, they're now standing at the boss spamming whatever and additionally taunt every X seconds. Yes, that added complexity to the game and totally is going to make this not boring at all. The problem is that I honestly couldn't think up much more in terms of mechanics that would keep the tank interested without having the risk of a wipe if something goes wrong. The tanks are the one spot in a raid where you can't really introduce RNG, design-wise. If RNG kills a dps, few will care as much as if it killed the tank. Imagine the outcry...
And part of the problem is that... to make it more interesting, you'll have to put more pressure on the tank. So either you bore people out of the class or you put so much responsibility on their shoulders that they prefer to not be the one to blame when shit hits the fan. Either way, you're going to lose a lot of people.
So what to do? How about you start treating good tanks with respect and don't take them for granted. For years and years people have abused tanks or ignored them. They felt undervalued. Everyone is raving about that Mage doing X dps while the tank did a superb job on the side tanking some of the hardest bosses around with stability and safety and nobody notices. That's an unthankful job, think about it and tell your next tank if he's done a good job. Chances are he'll return to tank another day.
As for Blizzard, they need to address this on a boss design standpoint. Meddling with the classes rarely fixed anything and only pissed the people off that prefered the class as it was. The difficulty is in finding interesting things for tanks to do to keep them interested week after week while at the same time not introducing mad rng factors that make them feel helpless. I can think up half a dozen things, but they're already in place in the game, so I'm out. You come up with something new, it's more difficult than it looks.
TL;DR:
People don't want to tank because of lack of recognition, boredom, too much pressure, no competition. Class fixes won't address those problems. Boss design may. Player attitude can't be programmed into the game, part of it has to come from the community.