3catcircus
Adventurer
If I follow this correctly, a battle against a dragon would play out as follows:
Maybe I'm missing something, but there's a reason why 5E went with scaling HP rather than scaling AC. When you make an attack, and it has literally zero effect, then the game feels boring. Not that I entirely disagree with you, and I definitely think they could have used a hybrid approach in order to better limit HP inflation, but the system you describe does not sound like much fun at the table.
- The mage hits the dragon with a level 1 magic missile, increasing its injury level to "slight" and giving it -1 to most checks.
- The cleric uses a cantrip, dealing ~3 damage, which increases the dragon's wound level to "moderate" and gives it -2 to most checks.
- The rest of the party attacks, firing any number of arrows and swinging their swords repeatedly; and they're just fishing for crits, because any attack that does less than 25 damage is going to be ignored.
- The dragon definitely kills the entire party, because it's literally invincible at this stage of the fight.
- Unless there's a rogue in the party, of course. Sneak attack bypasses invincibility, and the dragon will die in two more hits from the rogue, which are the only hits that matter.
The only thing you are missing is that the rogue also couldn't kill the dragon that easily - highly likely the only result (and the only result that deserves to happen from a 1st level party attacking an ancient red dragon) is a TPK. Invincibility?? A 1st level rogue doing a sneak attack is gonna only do an additional 1d6 damage. Assuming a crit on a sneak attack using a short sword, one could assume a Max of 24 plus STR or DEX - assume DEX 18 and it is a Max of 28 points. That would do a moderate wound which, combined with the existing moderate wound would put the dragon into a serious wound. A 2nd crit sneak attack would only do 28 points max - you'd need to do a minimum of 38 damage to inflict a 2nd serious wound to result in a critical wound. You could end up with a dragon that fails multiple CON checks from moderate or serious wounds leading to shock and eventually instability, but with a CON of 29, it's extremely unlikely unless the DM fumbles the roll.