REALLLLY helpful...
Well, it's not true (presuming a median of a sixish encounter adventuring day is met).
I understand from our prior discussions you dont bother policing the AD, so that's why you get the results you get.
I'd suggest instead of nerfing casters, you simply pull levers around the resting rules.
In my game, Short rests are 5 minutes long, with a max of 2 per Long rest, and each one no more than 1 hour apart. You can spend up to 1/2 your level in HD (round up) per Short rest.
Long rests are 8 hours long as normal, and you only recover 1 slot of each of levels 1-5, plus 1 slot of levels 6+ (your choice which slot). You recover no HP automatically, and recover 1/2 your level in expended HD (which can be spent automatically to heal).
Other than dragon breath weapons, not really. First, most (if not all) those effects allow saves for half damage (which are often made as well).
Yes, really.
Not just breath weapons,
all AOE attacks. Spellcasters, Dragon Breath, hits from Huge+ creatures etc. These are all incredibly common threats from T3 onwards.
Example: Fire themed dungeon for 14th level PCs (Quest: PCs must slay the Fiendish dragon by midnight or else a wave of Abashi are released into the Material plane):
1) Two Fire giants guard the dungeon entrance.
2) Marilith (AC 20, carries a shield, reduce attacks by 1, add poison stinger to tail attack for an extra 6d6 poison damage and poisoned condition, DC 19 Con save for half and no condition. While poisoned in this way, the creature is paralyzed, DC 19 Con save at the end of each turn to remove). Swords have the throwing and returning property (range 30/60) that only function for the Marilith
(short rest)
3) Erinyes and 4 Hell Hounds
4) Archmage and 2 Fire elementals
(short rest)
5) Half Red Dragon Fire giant, and 4 Half red dragon veterans
6) Adult Red Dragon (spell-casting variant). Room is desecrated; creatures in this room lose Fire resistance if they have it.
Most of those creatures can easily force one big (or multiple) Con saves for any caster than is unlucky enough to be targeted by one. And they are not 'unusual' monsters by any stretch at this level.