so we have two rules now? Why do we not have one rule for falling and it tells us whether a given fall is lethal or not (and if not, what the consequences are, e.g. damage taken)?
That would make more sense to me.
Yes, I agree. Once we accept a new rule into our game text, we very well could decide that some existing rule is no longer needed. That's not always the case. For example, using the falling example, my new rule could be
specific rule (lethal falls) applies solely to falls from above 200 feet
general rule (survivable falls) deals with falls unless a specific rule overrides it (drawing upon a metarule that already exists in 5e)
But I take you to be contemplating
rule (falls, lethal and survivable, any height)
It's really just a rule templating choice. I'm drawn to adding my specific rule over novation, because it sometimes turns out that a rule from the base text is doing something more than it appears on surface. So I just make the specific rule I need. That often makes it easier for others to know where they stand - "Yes, everything you read there is true,
and..."
On the other hand, there's no doubt that novation is sometimes the best thing to do.
Is that a matter of your design only, or is there some deeper reason why you added a rule instead of changing an existing one? Is it because now the rule decides what is lethal and what is not rather than the fiction?
The rule never decided that, truly. We decided that lethal falls for high-level characters are something we want in our fiction. We want to be able to say "Ha ha, it's from 1000 feet, you die!" and savour the schadenfreude that affords us. Or maybe it just offends our suspension of disbelief every time high-level Harriet HALO jumps without a parachute.
Whatever the case, the fiction came first. To see that, consider that we wanted a game about airbrush artists showing off their hotrods. Then it makes sense to build rules we can invoke around airbrushing, hotrods, that sort of stuff. But if our game is instead about hirsute Visigoths plundering Rome, we'd not design those sorts of rules because we've never invoke them (unless we're picturing hirsute Visigoth airbrush artists in hotrods, plundering Rome?)
Constitutive rules can open up new possibilities, and then we can design rules into that space (aka "design space".) Doing so at times casts light on new possible fiction. Otherworldly fiction, at times. That's why I have been neutral on whether falls should/should-not be survivable. While I say that the rule never decided that; in our engagement with rules they influence whether or not we accept them into our norms of play. To see this, picture a D&D feat that read "Rubbery: never takes more than 10d6 damage from falling, from any height." We could well accept that rule into our norms
even though it on surface breaches our possible feeling that falls from some heights should be lethal regardless of character level. It would probably be even more acceptable if we positioned it as trait of a new (non-human) race.