Yes, they risk losing some long-time tanks in hope of attracting new ones. Its the only real move I can see, because AS IT IS there are nowhere near enough tanks, and the situation will worsen at least temporarily.
If what you two suggest is true, that there is a certain fixed number of potential tank players that can only decrease with changes, but never increase - then the entire move would be pointless, and we are basically destined to have massive queue issues forever.

I dont believe it, others dont believe it. I know people who have never in their MMO life played a tank who pick up DRK now (ok, admittedly, its just 2 ppl, so purely anecdotal) because they like the cool look, but hated the overloaded busy playstyle early on.

SE cannot afford to just shrug off the waiting times. They have to do SOMETHING. They (and me) believe that making tanks more complex and difficult is SURELY not attracting more players. They (and me) believe making the act of TANKING harder and more challenging will probably make it even worse. The status quo is not good. So they try a change in the last direction they see for themselves, and see what happens.
You have to take a risk and change things when the status quo is not working.

If i could name a way to make tanks and tanking more attractive to players that I have seen work anywhere, I would. The only thing I have seen in other games is to make tanks super-simple to play, badass OP to play, with cool looks and lots of perks. The game is called WoW, and it has solved at least parts of its tank crisis long ago, by going the route we go now.