Justice and Rule
Legend
A intentional design point in 5e with bounded accuracy was that the primary knob for how long a creature lasts is HPs, no longer with defenses as part of the mix. However, what that means is that (a) solo creatures don't last long enough because they are also designed to be non-solo creatures vs. higher level PCs. Especially with classes that can nova or still do much of their damaeg with fewer hits (SA, paladin smite, etc.)
(Note: I don't think this is a bad idea. Bounded accuracy - which might be more accurately described as bounded targets - not only keeps things viable across longer level ranges, but also lowers what a +1 means - if you need a 11 or higher, a +1 will succeed 10% more often (50% to 55% -> 5% of 50% is 1/10th). If you need an 18, that +1 is means succeeding 50% more often. So maximizing every +1 is mroe important when you can be going against foes that need extremes to hit.
So in addition to the action economy they try to solve with legendary and lair options, they just don't have the staying power.
This is a difficult problem to resolve if the party is supposed to be able to meet these creatures later in multiples without mechanical changes.
Yeah, one thing I found is that with BBEGs, you want at least something else to take the heat off of them or act as a distraction because focused fire from a decently-prepared PC Party will absolutely overwhelm an individual endboss.