A given player "needs" a class because they want to play it. WotC "needs" a class because they are doing a release that really calls for it. There's a strong argument that none of the prior releases really needed one.
WotC set out with Next to prettymuch please everyone, impossible though that may be, so what the players 'need' (want to be able to play in 5e as they did in their favorite prior edition) is a legit consideration.
"Need" in the sense of a setting requiring a certain class/monster/spell/item mechanic because it figures into backstory or theme or whatnot is different, and could have had more to do with the order of setting releases...
Is Dark Sun going to sell as well as Eberron? Probably not. So, Dark Sun comes later.
It is a matter of opinion as to which is more central to the core themes of Eberron - artificers or a psionic race.
Eberron very overtly had a place for all things D&D, including psionics. Dark Sun, in contrast, very much did not (no gods, arcane magic restricted/changed), but psionics had a very prominent place.
If 5e was going to pursue such a slow pace of release, and keep psionics out of core, Dark Sun and Eberron, both, should not have seen print without psionics, while Eberron surely also needed the Artificer, Dragonmarks, &c.
I have no issue with inconsistency between the general class-philosophy of the PHB and supplements. If that inconsistency sticks in your craw
When consistency with a design philosophy is used as an excuse, while not stuck to in other cases, yeah, it kinda does.
Nor is it strictly a 5e thing. When 4e fans noted that a Source wasn't 'complete' - didn't have all 4 roles represented - MM bristled "we're not grid-filling" then went on to fill the grid of every other source.
I'm sorry, but he universe is under no obligation to meet yoru personal requirements for elegance.
WotC isn't the universe.