To quote general Sherman: "War is cruelty."
if the choice comes down to leading your platoon on glorious but futile charge up against a box of concrete and machine guns and making the poor bastards, who are trying to kill you, suffer by turning their defenses into an oven, well.... frankly i hope our armies make the choice that keeps their men alive.
That having been said, they are never going to come back as a conventional weapon, not for a moral reason but for practical ones. A flamethrower's range sucks, fifty yards at the best, most marine can put a bullet between your eyes at a few thousand with their iron sights. They weigh a ton. They have a horrid survival rate since if your carrying one you get sent into the most dangerous circumstances, and since everyone fears them all of your enemies will be gunning for you. Oh, and lets not forget, they tend to explode.