I have always disliked insta-death boss abilities in MMOs. It is simply a cop-out for lazy design. When it is difficult to design challenging fights with increasing gear level (meaning higher HP, DPS, etc) and player skill, lazy designers just throw in "death-touch" abilities.

If anyone remembers the old Everquest days, bosses would just randomly target players and "death-touch" them. The only difference in FFXIV is that the designers disguised the unpopular "death-touch" as avoidable Titan/Leviathan knockout abilities and Melusine petrification. Such abilities totally ignore gear level and character stats and eliminate players from the battle.

No matter how much you disguise the archaic "death-touch," you can't fool the player. Design innovative ways to challenge higher geared and skilled players without insta-killing them. Otherwise, people will catch on soon enough on just how lazy the boss fights are... as evidenced by this extremely long post from the OP.