You don't "need" any particular class, concept or archetype for most RPGs. Well, I suppose if your game is "Wizards & Warriors" you would need wizards and warriors but in a more generalized system, such as D&D, nothing is essential. Classes are merely packages of features designed to emphasize a play style or thematic archetype. Include only the ones you find relevant for your game.
The OD&D thief started as a class that interacted directly with the xp for gp advancement system. If the goal was to gain gp to advance, well the thief was designed to extract as much gold as possible from the dungeon via avoiding treasure traps and accessing rooms and chests containing said gold. Sure other classes could do this but thieves were specialists at it. It was their sole reason for existing and, in fact, their specialization came at huge costs in fighting power and survivability. You wouldn't confuse an OD&D thief with a fighter in leather armor and high DEX. They had entirely different capabilities.
In later editions, the thief remains as a legacy class even though the gp advancement system has been largely discarded and the archetype has broadened to "skill-based, high-DEX, lightly armored" hero (more rogue than thief). Their combat effectiveness has increased to often be on par with (or superior to) warriors even as activity in the exploration pillar been de-emphasized.
Can you dispense with the thief class in any particular game? Yes. Can you give thief abilities to any other class, say fighters, and call it a day? Sure. Or could you turn them into feats for any character to optionally pick up? I don't see why not. It really depends on what options you want to give your players. D&D opts for class packaging for this sort of thing and I don't see it changing any time soon. That doesn't mean it's the only way.
You might decide that a quick, sneaky, lightly-armored skill-based character shouldn't be a viable option in a particular game and that there should be no character-building pathway for this to happen. For example, a game centered around honorable, armored questing knights ala the Round Table would have no need of the thief, save as potential adversaries.
On the other hand, you might decide in a dark, urban Blades in the Dark-type setting that all characters are thieves, regardless of their actual class, and gain abilities accordingly.
Is "thief" a strong enough concept to maintain it place in gaming? Yes, as much as "fighter" is, or "cleric" or "wizard".
