Apologies but this is just wrong. The Darth title became a title was conceived way back in gosh darn 1978 when Lucas decided that Vader was Luke's father and his name was therefore Anakin Skywalker.
This exactly. And it's all because of the prequels. Before they came out, it seems like most of the Sith Lords in the EU were not Darths. They had names like Naga Sadow, Exar Kun, Ulic-Qel Droma, Marka Ragnos, Tulak Hord, and Freedon Nadd. Then Lucas came out with Darth Sidious and Darth Maul and suddenly the idea that Darth was a title and not Vader's first name became prevalent throughout Star Wars canon.
BioWare greatly added to this with KOTOR. And the lore hasn't been the same since. I've always maintained that the title Darth should only be reserved for the leader of the Sith Order, such as the Emperor and his Wrath. Everyone else would be Lords.
By having so many Darths running around it completely dilutes the effect of seeing one. A Darth is the face of darkness in Star Wars and the main evil - The Enemy so to speak. But now it's just a title that basically just means "General".
The meaning of the title Darth really hasn't changed fundamentally since it was first conceived. It is and always has been a Sith title used by certain Sith Lords that precedes their Sith name. And it always has been a symbol of power and supremacy over others and over fellow Sith. All the EU has done is explained how the usage of the title has changed and evolved over millennia, always fundamentally being the same thing, but used in different contexts.
And that seems pretty swell to me, history is not static, you can't expect everything to always be the same. For example this idea that the Darth title has been "diluted" is exactly what Bane believed and what started the Rule of Two.