This is an argument I see leveraged in opposition to positionals in many of the threads that have cropped up about positionals here. Even popular streamer Preach, in his recent stream about the announced Viper changes, argued that one of the reasons positionals are so disliked is the way bosses can prevent you from doing them, or a bad tank might position the boss in a bad way that makes them hard, or even impossible, to land successfully. The argument more or less goes: "Since these things exist outside of my control, they make positionals unfun to execute because even when I'm trying to hit them, sometimes I miss and it isn't my fault."
While that feels like a fair stance, I encourage you to look at . . . any other role in the game? You will see they are just as susceptible to interference by other players as melees are.
For instance, if I'm playing caster and an inexperienced Tank points a boss that happens to have a cone cleave AoE attack, I might get hit by the cleave and have my cast cancelled by the damage. My only option is to move out of the way -- potentially losing DPS to do so. Or, if I'm on a high damage caster like Pictomancer and a tank dies, I now hold threat from the boss and its autos can cancel my casts or kill me. But it wasn't my fault, and there's nothing I can do about it except die or wait for some tank to provoke. So should casting, or perhaps even tanking be removed?
Or maybe I'm on Healer and I'm in a burst phase, but a melee DPS accidentally soaks a mechanic. I might have to drop a damage GCD in order to heal them or they'll die, even though it wasn't my fault they got hit. I could just ignore them and let them die, but it's objectively the wrong decision to favor my one damage GCD over the life of a melee DPS. But why should I have to heal them at all, if it wasn't my fault they took damage? Would removing healing prevent this frustration?
Obviously, these hypothetical removals are ridiculous, and I hope this illustrates why arguing that positionals need to go due to how other players can impact them is similarly ridiculous. This is a Massively-Multiplayer Online game, and other players of course have the power to impact us for better and for worse. Sometimes that can be annoying, but we shouldn't remove casts because Tanks can break them and we shouldn't remove healing because DPS might take extra damage in certain parties. Therefore, it also seems fair that we shouldn't remove positionals because they're harder to execute with certain tanks. I mean, compared to the other proposed damage losses here, losing a full cast of a long-cast spell can be several hundred potency, but a positional is 50 or 60 at the worst, so melees are on paper even less punished than Casters or Healers in this regard.
Hell, eliminating all the bad also gets rid of all the good. For every time I've had a Tank playing DDR with the boss, spinning it all over, I've also had a skilled tank that always keeps it turned exactly where I want it -- especially in high end content where it matters. I'd hate to be denied moments after the duty where I can go "Man, that tank was good!" simply because sometimes, a tank was lacking.