I've told you repeatedly that I don't think this edition is flawless.
Quick, name 5 flaws of 5th edition. Actually, start a new thread and expound on them at length.
;P
...on to the actual topic...I pulled out these three quotes because they're all suggestive of what the Power Words were, originally, and maybe aren't so much anymore (or are they?):
The spell is useless for player characters.
It shines when an Archmage, say, uses it on level-appropriate heroes.
According to legend, EGG did originally conceive of the higher level spells (6th and up, at minimum) as tools for powerful enemies, and I suppose it shows. So if that's how it shakes out, now? Cool, classic feel ftw.
A single word to potentially drop a weaker target or finish a wounded one is rather convenient. And also, kinda cool.
To me, power words have always been about the casting time: whereas other 9th level spells are slow-casting, power words cast as quick as a first-level spell. But, 5E's default initiative system discards casting times, making power words kind of nicheless
Casting a spell in melee was hard back in the day, heck, casting a spell at all wasn't exactly carefree. You had to be standing still, upright, both hands free, etc and continue that way uninterrupted for, generally, 1 segment/spell level. A segment was six seconds. Think about that, you're going to spend 45 seconds performing a series of gestures so exacting and elaborate that you couldn't perform them if you were wearing leather armor? While someone takes swings at you?
But Power Word Kill is /one word/, so go right ahead and spit that in the face of the first sucker to run up to you (the first one to get through the walls of ice and fire and the prismatic sphere, anyway).
So, if we do want to rehabilitate Power Words, I think the place to look is casting time, and that prettymuch means a bonus action, or maybe even a reaction, to cast it, because it's just one word, right....