There really is nothing left to say than
There might well be no boss fights in D&D, but there damn well should have been.
I believe it is a disgrace the simulationists have successfully prevented real solo rules in 5th edition. They should have been present as an optional variant in the DMG, as a template perhaps.
And by "real" solo rules, I specifically mean rules that make the game work even if that means the Solo has to blatantly break every character creation rule in the book.
A true solo needs three things. D&D currently only supports two, and rather simplistically at that.
1) a Solo needs hit points. While Demons and Dragons have plenty of hit points, there is still no distinction made between Solo Dragons and non-solo Dragons. And there is no support at all for a humanoid NPC solo. (That is, there is no rule allowing a BBEG human wizard to sport the 500 hp or so he will need to function as a high-level Solo).
2) a Solo needs actions. I guess this is the part where D&D offers the best support, since there is at least one precedent for a creature being able to take an additional action after every hero's turn (even if there are as many as five heroes). I am, of course, talking about a Solo with five legendary actions per turn (Tiamat). Just change the mechanism of "Legendary Actions" from defaulting to 3, to defaulting to "one after each hero's turn" to make the mechanism scale, regardless of the number of heroes present (remember, some D&D groups play with as many as eight heroes).
3) a Solo needs saves. Legendary Saves, however, is simplistic and crude. I would much rather have a rules framework that allows each failed save to have some effect, but where a legendary creature is governed by a "three strikes and you're out" guideline. For example: one Hold Person or Banishment spell just restrains the monster, and loses it its next Legendary Action. The second (successful) Hold Person or Banishment spell visibly entraps the creature, which is incapacitated by now, but still not permanently put out of action. Magical energies means nothing has line of effect to the monster. Only a third spell takes the monster out as per the spell (allowing everyone to whale on the monster in the case of Hold Person; or making the monster disappear so the party can mop up in the case of Banishment)
TL;DR:
Of course D&D should support real Solo encounters, where the Solo is neither required to be absurdly overleveled, nor required to bring along allies (making it a not-a-Solo)!