<p>People will always stick to their own beliefs no matter how refutable they are.
People continue to believe that Sharingan and Rinnegan are evolutions/devolutions/mutations/stages of each other or whatever, even though it makes no sense.
</p><p>It's all in the name, Rinne (Rinnegan) Sharingan (Sharingan, duh)
Hagoromo instead of being born with Rinne Sharingan, instead first awakens the Sharingan (in the anime in the least, which may not be canon, but still), which is clearly a lesser form of Rinne Sharingan and then he gets Rinnegan, which is clearly also delivered from the Rinne part of Rinne Sharingan.
But in no way or form does it go like: Rinne Sharingan > Rinnegan > Sharingan/Sharingan < Rinnegan < Rinne Sharingan.
Rather it goes: Rinne Sharingan > Sharingan = Rinnegan.
</p><p>Sasuke's eye is unique in that it can have both the Sharingan and Rinnegan doujutsu active at the same time. It's not a different or special Rinnegan, just Rinnegan and Sharingan at the same time.
Sort of a pseudo-Rinne Sharingan if you will. But even though one of his eyes can have both doujutsu active at the same time, it's not a real Rinne Sharingan, because both aspects aren't fully merged into one.
</p>