That would kill everything except tanks, which means the odds are also super stacked against every other job except tanks. Not a very good argument. And one second implies they can do that with only two skills, since there's a 0,5-0,75 animation lock between skills. My personal crit record is like 6,7k with a fully buffed Between the Eyes in close quarters. Can't do that twice in a row, tho, so...number seems a tad exaggerated.
The odds are stacked against healers, yes, because the point and purpose of a healer is to make a game boring and stale by creating a perpetual stalemate of maxed HP on all players. Hence, games with all healers tend to be rather boring and only end thanks to the timer - the game doesn't work when everyone is actively working against it due to their role. Hence, culling time, vulnerability stacks and limit breaks were developed - those are the actual odds stacked against the healer. By design, they have to be denied their endgame and those mechanics were put in place to ensure just that.
Moreover, it says a lot that "playing correctly" in this context always means:"Countering the healer" via coordinated burst. That's not a given in game design, that's only contextually true because healers are so powerful that nothing but burst matters until culling time, as healers can undo everything else. In fact, when you look at job viability in PvP, the main criterion for viability seems to be how well and frequently you can counter the healer.
Summoners are a good example - Truckloads of damage, but so slow that healers can react, hence not favored. MCH - less damage, but so quickly that a healer has trouble reacting, hence meta pick. You name melee Limit Break, but ranged LB actually does more damage if it hits two or more targets. Why is melee limit break considered so powerful or even an issue then? Because it deals the damage to 1 target and thus gives the healer less opportunity to react. That's how powerful healers are, to twist and twirl the entire meta around them.
No surprise that every group wants to have at least one healer. Consequently, I, at least, never saw anyone beg people to please please switch to Monk or Machinist to stand a chance but frequently see people do that for healers. Their only weakness is that since by design, they are working against the winning condition of the mode, stacking them can't lead to success, because it leaves nobody working towards the winning condition of the mode, which is killing. That doesn't take away from their power however, it only shows their basic design flaw.