Consistency is what makes a setting believable. Without it, you're lost.
Where to me consistency and believability go a long way toward making the game functional and playable at all.
I did say they were important. However, consistency is not synonymous with believability.
Imagine if you will a hypothetical game where characters have 100 HP. Falling causes 1 damage per 10 feet, so any character at full HP can walk away from a 990 foot fall with 1 HP. Is it consistent? Absolutely. Believable? Hardly!
In the end, the game part is very important, IMO. If you put consistency and believability uber alles, then you're better off playing without rules. No ruleset is going to model reality to 100% believability. Heck, the dice are there to create inconsistency and will certainly introduce unbelievable events from time to time (the character who rolls a Nat 20 and pulls off a 1-in-a-million stunt). If you really want consistent and believable, then just have whatever makes the most sense to you happen, no rules or dice needed.
For you, perhaps. Not for everyone, regardless of what the marketing blurbs might say; and one of the true beauties of the D&D system is that it can and does handle lots of other styles quite well.
It does, but I was referring to RAW. 20th level characters can challenge gods and beat up giants for their lunch money. Even by 3rd level they're routinely taking on massive creatures that should be able to smear them into paste with relative ease (ogres). That's heroic fantasy, or at least a close relative thereof.
Certainly you can fiddle with the settings, such as capping levels, or simply not playing high levels, to modify the default. 5e makes that very easy.
I agree, and it doesn't make for a functional game if every level 1 character sustains a crippling injury from every attack. Scaling thresholds don't work, because low-level characters have too few HP compared to the damage of low-level enemies. If you set the threshold at half of maximum, then low-level characters may suffer multiple lasting injuries per day, while high-level characters never have to worry about it. There is no fraction which is reasonable for both groups.
I never said anything about a percentage. I said that if you use a threshold then it should scale with level.
A low level character should have a chance to suffer an injury, even if it's only on a crit. A high level character should not suffer the risk of injury from an average hit. Doing otherwise isn't believable.
There's a table in the DMG that indicates monster damage relative CR. The way I would do it is to base the threshold on the high end (maybe even 150%) of potential damage from a creature of CR equal to the PC level.
This way, there's a risk of it for any character, and 20th level characters don't have to save against taking injuries every time they get hit.
Personally, I wouldn't (and don't) use a threshold at all. I don't think it's a great fit for a game with HP scaling like 5e uses. You still end up with the issue of a high level hero whose arm can't be broken by a guy with a club, but at least that's reasonably in genre for a game where a high level fighter can take on a dozen low CR mooks and win without breaking a sweat (much less his arm).