The Robin Hood example kind of exemplifies the problem with the warlord, in that if I asked a group of D&D players what class would be Robin Hood's, they would say "ranger," not "warlord."
D&Ders, maybe, because they've been hearing 'Ranger' for 40+ years. For other folks, Robin Hood would /be/ the Archetype. They'd more likely say 'Archer,' 'Outlaw' or even 'Bandit.' And, if you insist on definitions, Warlord fits, too: he's got his band of armed followers, and a territory he's carved out of a state with a weakened government.
But, the ranger really only became a remotely appropriate Robin Hood, in terms of being a good archer later, in it's first two incarnations DEX was one of the few stats it didn't have a minimum for, getting a high-DEX ranger who might actually hit a little better'n average with a bow was a pretty wild stroke of luck.
The word "warlord" makes people think of tyrannical leaders of soldiers, like the daimyo of Japan or the more modern examples in the Middle East. It is not exactly a word that fits well with people's conception of a D&D character.
Or it makes them think of John Carter, Warlord of Mars. Or the 'evil' warlords Xena was always fighting. It's the kind of class name that flows in a fantasy milieu -
unlike Fighter (which conjures images of boxers) or Cleric. Even if warlords can be badguys in some of those sources - so can Warlocks, Sorcerers, Wizards ("evil" fits naturally in front of those even moreso than it does warlord). It sounds archaic, and connotes rebellion or at least independence, rather than legitimate rank, authority or position - especially compared to things like Captain or Commander or General. And Marshal - it makes Americans think of old-west lawmen. C'mon!
Back to the Dragon Highlords, yes they make more sense as Epic path things. It's kind of the paradox of a warlord, that it doesn't really sound like something a level 1 PC is supposed to be.
Hey, Wizard was an 11th level magic-user back in the day Gandalf & Merlin hardly scream 'first level,' Paladins were Peers of Charlemagne (the dozen highest-ranking/most powerful nobles in his empire), Druids were revered elders, priests & lawgivers in a tribal society often held in higher esteem than 'kings.' It's a completely spurious objection that applies to just about every concept getting the D&D zero-to-hero treatment. "Warlords" though, even by the modern definition, can range from leading little more than a gang of bandits to virtually ruling a country.
5e
does lack anything like Epic Destinies/Paragon Paths or Prestige Classes, you get a Background, at first, and a sub-class no later than 3rd. That's it. There's no high level distinguishing options or story/setting tie-ins like 3e & 4e had.
Prestige Classes would be helpful in bringing any setting to life, giving players ways to buy-in to the setting and forge ties with organizations, societies, cultures, or traditions within it - and could be used for high-level distinctions like the old name-level threshold.