Opeth has none of the characteristics of Death Metal except growling vocals. You can't base genre classification on vocals. This has nothing to do with "elitism."
Genre debates go nowhere. It's always a war between those with strict definitions and those who condemn exclusive classification. I'm moderate on this debate, and as evident in the past with regards to you, another battle need not be fought. Believe what you will, we all feel much more at ease without the Genre Police on every corner.
Based on my experience with metal, I can relate Opeth to death metal just as much as to progressive metal. You have probably heard the term "progressive death metal" used by fans and experts alike. There is your answer.
Then if death metal bands used progressive riffs occasionally that would constitute as death metal with a progressive influence. Opeth has just about zero death metal influence other than the vocals so they simply fit in easier with progressive metal.
You keep saying that there are other things, but the thing is that you're not saying what those other things are, and that's what we're looking for. Until you say what other characteristics of Opeth's music constitute the Death Metal qualification, this conversation can go nowhere.
Death metal is defined basically by its enhanced focus on rhythmic interplay. Opeth doesn't demonstrate rhythmic intensity nor do they exemplify any kind of dynamic in their "groove." They're much more focused on drawing out sections (which is something in common with black metal, however they don't really use any thematic phrases or anything, just a lot of drawn-out death metal riff construction).
Ever heard of variations on a theme? Opeth do not adhere to the basic principles of death metal, therefore they cannot be called simply death metal, but close enough to appeal to many death metal fans, which is the real reason why I suggested Opeth earlier in this thread in the first place.
Forget it. The point of this thread was to reccomend music to a death metal fan who wants some emotion in his music. It is a given that Opeth appeals to many death metal fans. It is also a given that Opeth's music is emotional. Put the two givens together, and you have a good reccomendation to offer. That's all.