Which is where verisimiltude gets thrown out the window
Only due to unwillingness to make it work
If the monster is epic, make it CR 17 or up.
Don't dumb it down so you can put it in a level 1-10 adventure.
If they choose the path of stupidity to fight the PCs on their own instead of using the potentially vast resources at their disposal
No
The strongest vampire in 5e is CR 15.
That's the problem. Elder Vampires should be more powerful.
I don't remember a single Giant or Titan in 4e being a Kingdom threat
Then you forgot. Giants go from level 13 to 24
Storm titans are above level 25.
Often in D&D what constitutes a "World threat" or "Multiversal Threat" is simply pompous hyperbole devoid of taking in 'the big picture' - I like to think this is a trick played on mortals by the gods for their amusement
No. It's not a trick
A street level threat is a threat to a street.
A city level threat is a threat to a city.
A kingdom level threat is a threat to a kingdom.
A world level threat is a threat to a world/plane.
A minor gang boss is just a threat to a street. If the city guard wanted to take him or her out, they could take him out
A major mob boss is a city level threat. It means they threaten a city. A city's power players are threatened by his existence.
The problem lies when someone says the Elder Vampire threatened the country. They can raise and command an army. BUT they are so weak they are obviously local
Dracula controls a country and threatens a continent. If you want to make a monster that is like a Dracula, it should be high level enough to do so or using a McGuffin to elevate itself.
The issue is people lie. If the world is extinct you better be above level 15 and that's the very minimum..
WOTC and many 3PP writers don't know how to write "If we don't stop X, the World as we know it will end" adventures.
Everything beyond the kingdom red level requires
whole new style of adventure.