I still slightly prefer the simple "Purgatory" mechanic, whereby would-be fatal damage prevented places a heal-absorb shield on you that must be cleansed (by healing) before Living Dead ends. Total Purgatory generated cannot exceed your maximum HP, which is still a faint gear-penalty, but during this the Dark Knight's self-healing is increased based on missing HP (flat, not percentile), which can ultimately overtake the penalty (if we'd just finally give Souleater significant healing). Walking Dead pre-immunity period would still remain and the CD shortened to compensate for the greater possible healing penalty, although on average this would actually mean a far lesser penalty.
But, I like this too. This is pretty intriguing in that there are many ways to fail it but it also means healers can just shrug and leave your fate to you alone. I don't know if it'll much more intuitive than what we have now, but it is interesting, for sure. From a gameplay perspective, I may even prefer it. There's that whole risk-reward factor of "How much can I self-heal for so I'm not AAed to death in 11 seconds but also don't end before the tankbuster that's also coming up soon?"
Let's be fair, though: there is a wide gap between the maximal effectiveness of Walking Dead and its intuitiveness or polish.
In the latter sense, it's absolutely horrible. Its UI sucks. Its tooltip sucks. The fact it penalizes you for being more geared sucks. It is dog**** and has deserved additional work since the day it was created.
But its not an ineffective ability. Regardless of pretty much whatever healers you're taking, it's ~9 to ~18 seconds of (not of "immunity", exactly, but...) scheduleable healer action. From the moment it truly starts (which should be a far more obvious transition) you have exactly 10 seconds to heal for max HP. As a healer I can enough easily guess out which non-crit oGCDs that will take just by knowing their max HP and what my abilities do on that. (At least unless the DRK gets hit by infirmity, in which case... rest in piece.) Given that I would still be healing up at least nearly that much had it been CDed instead, that's no huge loss to healers. Obviously we could have spared those resources if Hallowed Ground had been used instead, but then we wouldn't be able to have that extra 9 seconds or so of lenience; the "I won't die so long as I get heals" still lasts for up to 19 seconds, after all, and that's something we probably shouldn't so easily expense with when considering how to fix Living Dead.
Why hasn't it been removed? Because everyone needs a herp-derp immunity of some sort, I guess, and DRK's is still effective... enough?
Why hasn't it been changed (the obvious real meaning of the title to those who read the whole post and don't take it too literally)? ...Yeah, I don't know.