Drained reduces max HP and can last multiple days after being inflicted.
It's also a fantastically rare condition.
Various afflictions can cause enfeebled, clumsy, etc can similarly last multiple days. These cannot be removed without a 3rd rank spell (which can fail) or multiple days of recovery.
I've never had a character fail a save on a disease or other affliction.
Any PL+5 single opponent will be nearly mathematically impossible for a normal party to beat.
Well yeah, I can kill a party. It doesn't make the challenge interesting.
Enemies can have skill profienciencies. See the horned dragon above - it has very high proficiencies in stealth and deceptions + knows the Humanoid Form+Vanishing Tracks spells.
I guess if you make it a spellcaster, it can be a little more interesting.
But let me ask you, if you've ever played/GMed a Pathfinder adventure, which should highlight the very best designers in the game using the system as it is intended, which of the two following scenarios sounds more likely?
1) The party has been traveling for days through the primeval forest. They have noticed elk and other large game impaled as if by a massive spear, but there's never any evidence of something having moved through the underbrush. Here and there they see bark ripped off trees, almost as if a claw was carving runes in the trunks. As they travel more deeply into the ancient woods, it is becoming clear that they are in the hunting grounds of a terrifying predator.
When camping one night, the ranger catches a glimpse of a logger whose attire is torn and bloodied. "Gods ... help me ..." the man begs. (this is the dragon in humanoid form.)
The ranger steps forward to investigate before being separated from his companions by a wall of thorns.
The logger laughs and reveals his true draconic form. Taking to the air, it sprays a cloud of poison on the sleeping party. It flies off, the party coughing and attempting to put on their armor. When will the creature return for another pass? Will they remain sitting ducks out here in the woods? They must travel to find shelter against this creature or force it down to fight.
OR
2) Here's a glade battlemap, approximately 20 x 20 squares. These are the squares where the dragon starts. Roll initiative. The dragon fights until dead.
So I will posit that the designers don't intend you to play the game as I described in Example 1. If so, that's what they would publish. And they would make these abilities default in the monsters. And they would give more lingering injuries, more difficult to treat afflictions, etc.