MNblockhead
A Title Much Cooler Than Anything on the Old Site
Good question. Never gave it much thought beyond thinking that monk should be a subclass of fighter. Another thought that I had that if D&D ever added psionics back into the game, the psionic class should use ki for its powers. All classes and subclasses are a bit arbitrary if you think about them too hard.Why oh why don't more people use ki in D&D worlds?
Ki exists, and provides an alternative means to perform exceptional (bonus action dash), superhuman (paralyzing a guy with a touch), and even magical (four elements shenanigans) effects, all using an internal power source that resets with 30 minutes of meditation.
So why are Monks the only people using this ability? At the very least, you'd expect ki-using subclasses to exist for other classes!
I'm already burnt out discussing this in the recent "So, how about alignment thread":Why oh why are so many players so vehemently attached to alignment?

D&D General - So how about alignment, eh?
I figure we could use a palate cleanser. So, how about alignment? Do you prefer the classic nine, the 4e five, or something else? What about Chaotic Neutral – 1e/2e's random wackiness or 3e/5e's anarchist?

In short, it is part of D&D's DNA. I would except it being relegated to the DMG as a variant rule, but would not want it to disappear from the game entirely. I use it more like a faction system at the cosmological level. But if you want the details, there is a current and lengthy tread on this.