I want to be clear: I'm not HAPPY we don't have a proper Warden, as it was probably one of the coolest new class of 4e (and I never got to play one myelf, booh!)Whereas for me I think the theme is completely out of left field*, and it--like the Oath of Vengeance being "Avengers"--feels like a slap in the face to people who actually liked the 4e Warden.
Like, if someone told you you could play a Warlock, and it turned out Warlock was just a Wizard subclass that had spooky fluff and one "talk to your patron" feature, but no Hex, no eldritch blast, no invocations, literally nothing like either the 3e or 4e Warlock....would you think "hey, awesome, they gave us something kinda-sorta-vaguely Warlock-like"? Or would you think, "Wow, this was the best they could do, huh? And they're still calling it 'Warlock.' Just wow"?
I'm definitely in the latter camp. It's a similar stance to my rather significant frustration with the "no no no never EVER add more classes, in fact we should have fewer classes and condense everything down as much as possible." That is, trying to shoehorn too many distinct ideas into a single framework very frequently results in one of three things: (a) diluting those distinct ideas until they're barely there (as is the case with Paladin), (b) diluting the base so far that it ceases to have enough stuff in it to be compelling on its own (e.g. I would say Wizard and Fighter are in this camp), or (c) a power-creep spiral (Twilight Cleric being a good example here.)
The purpose of making an actually new class, if a new class is warranted, is to carve out a solid thematic-mechanical package and give it the solidity and support it needs. Subclasses then offer the opportunity to specialize, diversify, or present interesting contrast.
*The Green Knight already exists. It's called the Ranger. The fact that many folks, such as @CleverNickName, see a better Ranger in the Oath of the Ancients Paladin is a condemnation of both things IMO, not a positive about either.
But I still like the 'Old Faith', slightly Celtic flavor of the Oath of the Ancient. It feels like a Paladin of an age past, that follows an almost forgotten god. The god an Oath of the Ancient Paladin follows is not one that most people have heard about, not because it's secret or anything, but just because of the lack of reach, maybe the lack of a need to proselytize? There's a certain noble loneliness to it.