Felon said:
Suffice to say, your "internally consistent logic" can quickly seem pretty spurious to a lot of folks. Every class of enemy isn't going to think like a dungeonmaster, and have a deep understanding of the adventurer's playbook.
Of course not. Humans in a civilized world, hobgoblins (militaristic and intelligent), very intelligent monsters (Illithids, vampires, dragons, drow, etc).
A pack of goblins or giants aren't. But those aren't going to have hefty spot and listen checks to begin with.
There's a gap between "magic is prevalent" and "everyone's familiar with PHB spells and knows all manner of ways to short-circuit spellcasting interlopers".
Who said anything about "Everyone"? I don't expect joe farmer to know the difference between fireball and flame strike, or even the grunt standing outside the door. I damn well expect the Lord's adviser to. I expect a captain of the guard to have a level or two in expert for skills like spot and listen in addition to Knowledge (Bla) or Profesion (Guard), and awareness of magical means to bypass security would go under Profession (Guard).
D&D is a game in a pseudo-medieval world where most folks' knowledge is limited to personal experience.
Any professional guards would have been trained to deal with magical-wielding foes just like guards before were trained to deal with stealthy foes. If the Lord can afford a small company of armed guards, he can damn well afford a wizard adviser who knows a thing or two about 2nd level spells. That goes double for any castle or warehouse with "high security" merchandise stored.
It's really simple. If everyone in the world knows that anybody can become invisible if they have the resources, then anyone with anything worth protecting will take an effort to prevent it.
If people in your campaign world have an idea of what a beholder can do or a troll can do or a dragon can do simply by stories and legends, then they should have a general grasp of what a wizard can do.
NPCs taking the means to deal with invisible burglars is no different than current-day establishments using security cameras. Because a skilled thief or operative knows how to deal with them and bypass them.
If you want to play in Arthurian-land, go ahead, but then don't have wizards and sorcerers in your game because spellcasters throwing around meteor swarm isn't appropriate to the game, and neither is throwing invisibility all over everyone. D&D as it stands isn't equipped to deal with low magic worlds, because everyone needs magical items to function at levels beyond fifth, let alone the power and flash of spellcasting classes.
And if Invisibility makes sneaking into anywhere a cake walk, then having a +8 to MS and Hide must make it a cake walk too. Because any place that can't equip their guards to deal with invisible foes can't afford to have guards with antyhing more than a +3 to listen and spot.
If I'm sneaking into some place bigger than a bandit camp or goblin cave, I
expect the DM to run it like I was James Bond infiltrating enemy headquarters. Because just slapping on an Invisibility and strolling in is, in my opinion, not fun at all.