Defense on necromancer has always involved soaking up damage rather than fully mitigating it. This is why necro has access to a larger health pool and life force, with no access to effects like blocks, evades, and invulnerability. This also means that necros defense is inherently worse than those of other classes. In any game, ignoring damage is better than tanking damage. That doesn't mean necro should get access to blocks, evades, and invulns. It means necro needs to do a better job of tanking damage. Necromancer has a very unique play style. Proper life force management can make shroud a very effective defense. Adding blocks, evades, or invulns to necro would remove its identity.
The problem with tanking damage is there is currently no disincentive to continuously attacking. When any other profession puts up their defense, the reaction is to stop attacking or at least not waste any heavy cool downs. This reduces pressure on the defender, and provides windows for counter-action to take place. For a necro there is no reason to stop attacking even if they drop into shroud, hence the punching bag meme. Maybe in the early days of GW2, damage was so low that the large health pool provided enough survivability to exhaust cool downs and count as an effective defense. However with damage as it is today necro relies heavily on terrain and kiting for survivability, which makes skills like Spectral Walk and Flesh Wurm mandatory.
What necros need is a way to disincentivize attackers from unloading everything and 100-0ing them. In games like DoTA2, successful tanks will build Blade Mail when they can no longer sustain the damage they are tanking. If necro's defense is it's large health pool, then they must get some way to disincentivize attacking for it to be an effective defense. I suggest the following changes to the Death Magic specialization:
If necro still feels too much like a punching bag, then the damage from dark defiance and retaliation can be adjusted, or additional toughness could be added to Diabolical Fortress similar to how ferocity is added to reaper's onslaught/death perception to make those traits more effective.