The standard for "causing the internet to explode with controversy," is ... people on the internet.
Addressing the actual article-
(1) The Witcher is considered successful at Netflix. Any show that Netflix makes that gets three seasons and is renewed for a fourth is a success.
(2) The showrunner specifically avoided hiring people that were too familiar with the source material- in other words, it was a feature, not a bug. This is not an uncommon approach; famously, Alfonso Cuarón did not read the book Children of Men. But there is always a push-pull when adapting books (or video games) to a new medium; you have both the problem of translating to the new medium as well as (almost always) finding a way to make the material accessible to a broader audience (cue up "fan service").
(3) Henry Cavill has not said anything about this in regard to leaving The Witcher; however, it must be noted that this announcement came at the same time that he announced he was returning in the (big budget, big screen, big time) role as Superman.
He made a business decision- likely a lucrative one. While I didn't like his prior Superman movies, I definitely like Cavill in the role and I am excited to see what he does with a new direction.