While it's easy to make a Warlord-lite sub-class or psionic-whatever sub-class, that's not the same thing as doing justice to the concepts with /just/ a sub-class or even a series of them across several classes. A figher sub-class, like the Battlemaster, if it had 10x as many of the right kind of maneuvers instead of a few token attempts, might do for a Bravura Warlord: one that's supposed to be a personal-combat badass who mainly just leads by example. You could fairly easily make a modest psion out of a Sorcerer origin or even by just reskinning a GOO Warlock. But, that's just a token effort. There's a half-dozen distinct Warlord builds you left out, and about as many psionic classes spread out over 3 editions. That GOO Warlock isn't going to make much of a Psychic Warrior or Ardent, for instance.
Making the Warlord a subclass doesn't take away those builds if they handle it correctly. There's easily half a dozen builds with any subclass right now. You don't really need another full class for what amounts to a niche role.
And psionics... a series of subclasses for each class could solve that. Psychic warrior? Fighter subclass. Wilder? Sorcerer subclass. Psionicist? Wizard subclass. And so on. You could even come up with entirely new psionics options via this method while still keeping it to the current class system.
Plus, they really do need to flush out the main classes with a lot more subclasses before we're even close to those having enough attention.
Maybe you don't feel the need, because you just don't want the classes, themselves, but it's a very clear need. Trying to run Dark Sun by faking psionics with one sub-class just isn't going to cut it, for instance, and there's whole playstyles and player choices the Warlord opens up just by making a non-caster support class viable.
What very clear need? I've seen these arguments a hundred times before, and they all amount to what people
want without a true justification that it's actually
needed.
Personally? I would love a psionics class... but I can't justify it. I can use the same arguments people use for a Warlord... and, at the end of the day, have never justified adding one at all.
No, there is no very clear need. The system they have in place eliminates most of the necessity, and careful balancing of new additions as subclasses will eliminate the rest.
And, before someone says anything about giving people what they want being a good idea: WotC has already learned from that mistake. Listening to people on forums is what produced 4E, and we all know how that edition turned out. So, yeah, if they're smart, they'll put forum topics like this right on their ignore list and continue listening to the surveys. Which it appears they have. And, hey, guess what surveys caused the lack of Warlord in 5E?
Well, the PrC genie (there may even be a Genie PrC

at some point) is out of the bottle - that's where the really crazy class bloat was in 3.5 - so that slippery slope is already pretty well greased as it is. I doubt trying to hold the line on a legitimate full class or two, like the Warlord (in a prior ed PH1, opens up playstyles 5e doesn't support yet, has 6-8 potential sub-classes) or Psionics (in the 1e PH1, in every edition in some form, including 7 or so distinct classes), is going to make a big difference. The slow pace of publication means bloat will happen slowly, and you can always stick to 'Core Only,' as AL seems to be more-or-less doing (Core + whatever came out this season, but not for long).
Nah. The really crazy 3.5 class bloat is in Pathfinder. Plus, 3.5 had a growing list of base classes long before it ended.
And if they do PrCs how I suspect they will, those will also be limited in number with not much room to grow.
Now, where do I think the bloat will happen? Subclasses. They've pretty much set 5E up for that being the bloat this time around. The slow production time just keeps it from happening quickly. And, I don't think this is necessarily a bad thing. If handled right and kept slow, subclass bloat could benefit the game heavily in the long run. After all, are you ever going to say no to more options for your character?