Anyway, what Opeth does is (unless you wanna stubbornly stick to that idea of 100% originality, which is a beginner's attitude, an admirable one, but eventually inadequate) pure magic. Id like to know it all some day, which riffs are original, which are rip offs, and so goes for every detail in every song on every album. Id like to know it all, not for the purpose of spying or hairsplitting, but just for better understanding and putting the pieces together (give me some examples, ill be thankful, im a metal newbie, which Morbid angel's songs i should check to understand Opeth's influences better?).
So, to wrap this up, i think musical talent and quality of music cannot be exclusively measured by «originality» of compositions, nor can this originality be narrowly understood as mere usage of previously unused or unknown combinations of notes and chords. Originality and creativity is also in the art of combining the borrowed elements, having an ear for good ideas in otherwise average songs, putting them together in new unity, using them to your own advantage...
So, when artists dont talk openly about «stealing» from other musicians, i kinda lose respect for them as «humans», but i still judge their music purely by emotional energy it conveys. As for Opeth, they really wear their influences up their sleeve, so their integrity cant be questioned. It might be a little disappointing though to discover that Mikael didnt come up with all the ideas and so, in a way that makes him a human rather than omni-potent god

, but at the end of the day, my opinion is that «true» originality is in how you combine all the elements, original and borrowed, and make them co-exist.