Only the ranger is really lacking of the three.
Both the Druid & Paladin have strong identities from myth/legend/history that a 'mere' nature-priest or fighter/cleric wouldn't do justice too (if you even accept that the current classes do them justice).
(And the Barbarian is also pretty tenuous as a class - could be a Background. And the Sorcerer is a pretty weak effort, too, not doing the 3e vision of the class at all well, thanks to Spontaneous casting being ubiquitous in 5e.)
Your comment about the Sorcerer brings up a thought I've had regarding classes in 5E (and modern D&D).
I kind of feel like a lot of classes in 5E exist solely as a means to express a unique mechanic in the game.
I wonder if there is a chicken and the egg thing going on?
The Sorcerer is no longer necessary as Wizards get to spontaneously cast from their prepared spells. So they invent sorcery points to justify the existence of the Sorcerer as a class. Did the existence of the Sorcerer class force the creation of the sorcery point mechanic? Or was the sorcery point mechanic something that they wanted to be put in the game, hence it got stuck on the Sorcerer class?
Is this a situation like the board game Root, where there needs to be different mechanical interactions for each class? Why is this required?
What drives the need for a Sorcerer to exist when the Wizard class handles the magic spell-caster with tremendous versatility, already?
I'm ok with just the magic-user, but I do see why people want a little more differentiation. But why does the differentiation have to be so strongly expressed mechanically? Why does a warlock need a completely different mechanic to 'feel like a warlock' instead of a wizard?
Proliferation of classes, to me, really means proliferation of mechanics.
I often wonder what is the point? Why does D&D need to constantly add new mechanics? Isn't the idea of playing in an unlimited fantastic world of magic, monsters, treasures, and the unknown enough?