True, but audiences tend to only notice blatant changes, such as a new character or change in a character's fate. Audiences tend not to care when they are entertained.Movies rarely are adapted faithfully to their source material.
I don't think most people taking the "purist" stance would complain if they liked Tauriel's inclusion, and I don't think her gender is the issue. Maybe Taurien the male captain becoming Kili's drinking buddy would also be an immersion-breaker.
Speaking for myself, GoT deviations I LOVED were those in episode 10. They were blatant fate-changers, but done well, so I loved them. People who didn't share my opinion felt immersion was broken and complained. (Professional authors' opinions are mixed as on this board: GRRM loves the changes in GoT. Tom Clancy and Michael Crichton hated the changes in their work.)