Even 5e runs into issues if you try to rules-hack it against some of the fundamental system assumptions it was designed around.
okay. If I changed HP to a damage threshold, that scales very little, and replaced monster damage with a level based chart, I'm pretty sure I could make that work. It would change the play experience, but I'm confident I could make it do so in the way I intend it to. Same if I make every attack hit, and your roll just determines damage, while AC gives damage reduction. It changes how the game feels, but it isn't hard to see how it would change it and adjust accordingly, and either use it or not based on what the play experience is most likely to be.
Gonna have to disagree on this.
That is not a rules mod - it is just stopping play before the games inherent advancement progression bursts your genre bubble.
Pretty sure you wouldn't challenge it being a rules modification if it was in place for every campaign. If I just said, for my group, 5e ends at level 12, vanishingly few people would argue that it isn't a houserule used to create a different play experience from what the core book assumes.
Having to cut out 1/2 to 3/4ths of the game is not a very good example of 5e handling things without "any problems".
I want to play through the full 20 levels of advancement progression the game says it can do.
Not starter-set style cripple ware.
That's just a preference. A perfectly valid one, but if no one at the table shares that preference, or has no strong feelings either way about it, then it isn't relevant to modding the game in terms of how long it lasts.
And you aren't entitled to getting to play all 20 levels. Every campaign that ends before level 20 isn't breaking the rules.
I'm pretty much never going to run a 1-20 game, because at some point before level 20, the story of these PCs will have concluded, and we will move on.
The fact is that when you do things that really mess with 5e's 20 level HP progression advancement; it has a lot of knock on effects. Because it is something that is accounted for throughout the rule system.
The salient question isn't whether I can easily mod the game for broad consumption. The question is can I mod the game easily for a specific story. If that means adding damage to monsters but not PCs to make PCs feel fragile, or to scoop out HP entirely and replace it with a system of graduated damage thresholds where damage over a threshold has increasingly deleterious effects, and massive damage can one-shot you out of the fight, the game won't break. It will play differently.
Average damage remains the same, stuff like DR and resistance and THP work the same, but are more important. Healing would need some kind of conversion system that can be made into a chart based on healing by level converted into "X healing has Y effect", and that would be possibly the hardest part. I wouldn't bother unless I was making a whole game based on 5e, but if it somehow served what I needed for a story, it'd be work that I'd enjoy doing enough that I wouldn't mind it. Nothing wrong with not making the same choice, but I'm not going to pretend that 5e can't handle that sort of thing just because the process would be time consuming.
Retooling the full ruleset to not work that way is fundamentally re-writing large swaths of the game.
Which is defiantly not handling things easily.
5e is fairly modular and can handle a lot, so long as you are happy playing within D&D's design paradigms.
But even it has limits.
I never claimed it didn't. You've taken a general set of statements and responded to it as if it had been absolute. Everything has limits.