Cavalier wouldn't be doable with the design of the game.
Disagree ...
Right now im playing in campaign, where one of us play Cavalier Fighter / Ancestral Barbarian ... and while we have no mounts (i asume that was your point?) Cavalier is still quite usefull addition to party.
So it would be doable ...
Probably it wouldnt be 1:1 with tabletop ... but since Swasbuckler, and Bladesinger are not either ... i would dare to assume Larian wouldnt really mind that.

It would be doable. But one of the main things about Cavalier is the mount.
If Larian added a mountless Cavalier to the game, we'd see endless complaints about lacking the subclasses most defining feature.
Samurai wouldn't really fit thematically.
Disagree again ...
As far as i know, there is nothing odd about Samurais in DnD ...
Yes, i know we have that word tied in our little minds with feudal Japan ... but that is just same word for something entirely different. :-/
Yes, there's nothing odd about Samurai in DnD.
It is still however, tied to Eastern locales in DnD. Which... Baldur's Gate is not. Sure, a PC could be a traveller from the East... But it's still an out of place subclass for the setting of BG3.
Rune Knight kind of competes with Giant Barbarian.
How?
Ok, they both go big ... and?

Rune Knight is quite versatile subclass, while Giant Barbarian is weapon throwing elemental brute ... there is not much simmilarity.
Both are focused around becoming large and gaining bonuses for it.
They're not identical, but there's some considerable overlap in theme. Something they might have factored in when choosing which subclasses to implement (Of course, they could have picked something else for Barb... Storm Herald or Ancestral Guardian could have been interesting)
And if you really mean this as arguing point, then im affraid i have to insist that in that case Arcane Archer competes with Range Ranger ...
After all one old DnD joke says:
"What is best way to play a Ranger? Play Dex based Fighter instead."
Except "Range Ranger" is a playstyle choice, not a class one.
No part of Ranger explicitly is about using ranged weapons at all. Even their weapon focuses provides them with mostly melee options.
Psi Warrior is basically Battlemaster 2.0
Battlemaster is a dude who is really good with his weapon ...
Psi Warrior is a jedi. xD
Thematically yes.
But in practice, they're both Fighter subclasses that get a unique dice pool to utilize special actions for. It doesn't really bring anything new to the class, it's just "Battlemaster but with different skills"
Arcane Archer on the other hand creates this ranged based playstyle.
Also ... even if ... disclaimer: i dont agree with it, but lets say for argument purposes ... isnt Psi Warrior quite litterally and by deffinition the MOST FITTING SUBCLASS FOR GITHYANKI?

Not really. It can be an appropriate subclass for them, given their innate psychic abilities and their martial training. But it's not definitevely the "Most fitting" since there will still be a lot of variance in what any individual Githyanki is capable of (Some might have less potent psychic powers, some might have less potent martial prowess, some might have an affinity for the Weave etc)
Which is reflected in all the Githyanki we face in game. Not all of them are Fighters.
Echo probably being hardest to implement
Maybe ...
Still, it would be cool.

Aye, for sure. But still, "Being Cool" isn't enough of a reason to choose various subclasses.