"tank" meta is not what it's commonly thought it is. You can wear hist sap, mara balm, any combination of tank sets. A character can normally heal 1.5x of damage (with occasional bursts but then it's sustain race), so nobody dies 1v1 just by default if one of those opponents wants to. When you hear complains "u can't kill me 1v1" or "yeah only 2v1 u can" yeah by game's default, that's the actual point.
If a good player stands still like a target dummy he can outheal the damage of another good player (if he wants to and builds in such way). But he can not outheal, just by simple numbers war, the damage of another two good players. It will take a lot for full tanks build but it's a sustain race that nobody wants to bother with.

so out of the way with sets that people think they bring the tank meta, the real problem with "tanks" is line of sight. This game has a global ~1 sec delay of your position (and any action in general). You will often cast your animation and then it cancels because target is out of sight. It shouldn't have casted in the first place if target was out of sight, right? This makes running around a pillar impossible to get hit because the game desyncs your position against your enemy's attacks. Good players do this by anticipating enemy's movement on the pillar and going there ahead of enemy, to keep the "real" time in same place while in game the desync is tricked.

So just properly using the broken targeting with line of sight you can become "unkillable" even against 2-3 targets even in full damage build, hence the "he can't die but he one shots me", because u r not even taking the damage in the first place, but u r dealing it because your enemy is not behind a line of sight blocker. It's not about sets, these just set the time required for kill and not the possibility of it.

as of what could change, fixing the proper targeting and line of sighting would be hard work and maybe past the engine's capabilities, definitely not worth for a corporation that solves problems as it does now. So for now, band-aid fixes like it's commonly done.
Maybe narrow the corners of target sight blocking terrain and enlarge the players hitboxes, if spell is launched with enemy in sight then fire that spell even if meanwhile target had its position updated to be out of sight.

Just for the healing and survivability itself issue, Defile could be reworked to be like the Relic debuff, some more details about this theory here:
instead of having 8/16 % healing reduced, dealing direct damage with disease damage , applies to the target the Diseased and hence the Defiled debuffs.
These are not cleansable, Diseased lasts for 8 seconds, and while active it applies Defiled effect that grows every second by 1% to reduce target's healing received (capped 99%, to keep "healing received" effects).
So if you don't take direct disease damage for 8 seconds, you lose the Diseased debuff, and Defiled debuff is removed any time Diseased expires or is removed in any way. Getting hit by direct disease damage refreshes the duration of Diseased, so this makes Defiled not lose the growing stacks.
Let's say: i am getting hit, debuff starts, during 8 seconds of Diseased i reach 8% Defiled. I get hit again after 6 seconds instead, so for the next 8 seconds of Diseased i reach 14% Defiled. Now again i get hit after 6 seconds instead going towards 12%, so for the next 8 seconds i will reach a total of 20% Defiled. And so on if i fight constantly for 50 seconds eventually i reach 50% Defiled, and can go on.
I know what this looks like, defile sources op now and necros become meta. Well necros in pvp do need something right now but in this case Blastbones should become dodgeable because it would be full uptime dooming someone to death without choice, and Defile could be reworked to be accessible better than some niche debuff. Also sets that apply the Defile debuff would need rework to not grant full uptime without player direct implication, at least for pvp.
Applied by direct damage means no dots or procs, but enchants should work here, also work effects that specifically apply Defile should work to apply Diseased as if being applied as direct damage. This makes absorb shield more valuable, hence magsorcs gain some needed defensive advantage (offensively they are at max limit of power, shouldn't get any more).