While very true, the problem with this line of reasoning is that it does nothing for all the times when you want the BBEG to be awesome enough to stand against the adventurers' on his own.
The way the game caters to realism in that no creature suddenly sports five actions and five times the hp limits the fantastical scenarios the game can handle. And I guess I'm not used to D&D being too cautious a game to handle the real space-out scenarios...
There is no good solution if you don't accept the 4E notion of "solos" or boss monsters, because by the 5E dogma, you're limited to either bringing in reinforcements or making the monster of such a high CR (like, more than double the APL) that the encounter starts to break in other ways.
(Such as the monster being so much higher CR that its suddenly immune to the attacks of the heroes, just to give one trivial example. Perhaps a more worrying example is that there are no monsters in the CR 40-60 range, which I imagine would be needed to give my players (or Celtavian's) a sporting match at level 20)
The problem isn't that adding backup doesn't work (it does, it's very effective). The problem is that the game provides no good answer for the times when you don't want backup.