I think this is pretty fair.Here's the thing.
In most of the older settings, evil gods and major fiends are very similar. This is because they were designed just heads of villainous organizations. Evil gods usually were for organized evil churches or to provide a group of "barbaric humanoids" with a healer/smiter. Major fiends usually gave you secret societies, mystery cults, or raving madmen who were heavily tilted to a basic human emotion or vice. But ultimately, an evil god and an evil non-god was 80% the same in crunch and fluff.
They were not designed to have cosmology or religious conversation or debate in game.
They were designed that after you one group of baddies who worshiped one figure, the DM can pluck and drop another group of baddies who worship somebody else to fight next.
I'm a bit less persuaded by this. I think some parts of Planescape probably did some of what you're describing here (not all of it - qv Dead Gods and its Orcus priest). But I agree that somewhere along the way there is a trend in some of the materials to take the cosmology seriously as a thing in itself.When 3rd edition came along, it lived with the explosion of internet use. Lore began to be discussion around the world in faster speeds. And this created criticisms. D&D's popularity grew and the increased communication allow more people to discuss offical and homebrew cosmoses, religions, and pantheons. Fans began to discuss the tropes of D&D more and pantheons and gods began to change. Newer pantheons were designed for in game discussion. Old Pantheons saw members change. And the evil gods and evil nongods began to separate.
Needless to say, I think that 4e reconciles the two parts of your post.