Let's try to remember that this is a game we're talking about. I think it should be designed one way, you think it should be designed another way.
Not at all,
how it's been designed has already been decided for 5e. It's got classes, backgrounds, races & sub-classes. It's got spells & slots, overnight healing and maneuvers. BA and Adv/Dis. It's DM-centric and DM-side customizeable.
That affects how a given class might be implemented in the game (rather a lot, actually), but it's no bearing on whether there should be a given class, at all.
You are depriving me of "my" D&D by injecting an official warlord into it just as much as I am "depriving" you of your D&D by keeping the warlord out of it.
Strange, because I was never deprived of
my AD&D because there was a psioncs appendix in 1e, or a psionicist class in 2e. I just didn't use 'em. It was dreadfully easy on me, compared to making everyone else create their own takes of psionics.
Then why aren't you satisfied with a 3PP warlord?
Maybe, if there were exactly one 3PP Warlord, and it was from Paizo or another very well-regarded 3pp,
and it was generally acclaimed and broadly endorsed, it could be adequate. If I were only concerned as a DM, it wouldn't even take that, as DM, I have tremendous latitude, and if there were a
good Warlord out there, I could take the time to find it rather than make one myself. But, that's not the case, there's more than a couple, and they're none of 'em that well-regarded.
As a player, it's worse. You're not guaranteed to get to play the character you want, even if everything you need for it is 100% PH official, and not tagged optional like, say, feats or MCing. 3pp is getting pretty out there, the effort involved in finding the best 3pp option out there for you what you want is, itself, daunting, convincing a given DM to even look at it, kinda a long shot. "Settling" for a 'mere' option, rather than getting a PH-official class is already a big compromise from the player PoV - and it's a compromise with an unreasonable position, to begin with.
Contrast that to, as a DM or Player, declining an official option you don't want: as a player, you play something else you do want,
possibly, if other players aren't open to catering to your distaste, and alternate groups not readily available, putting up with someone else playing one.
As a DM, you ban it from your table. Done.