Let me ask this question. Why does slowing things down and keep it simple have to equal "dumbed it down"? Some bands can find it as creatively rewarding as playing the fastest and most complex stuff, depending on how much they care about it and clearly, Metallica cared about the craft that went into making the Black Album.
Why are you taking things on this so personally?
Saying that they "dumbed it down" is not insulting you. And the meaning behind that simply means they took a lot of the technicality out of the playing and emphasized melody more. And yes, they clearly cared about that. But generally speaking, every time I have ever heard someone comment about a record using "they dumbed it down," to explain it, that's exactly what they meant.
I get it, you liked the Black album and the 90s stuff. That's totally cool. But why can't you just accept that Metallica very purposefully took their fast, technical sound, and slowed it up and made it more streamlined (aka "dumbed it down") so that they could appeal to a broader audience (and thereby cross over and bring in more album sales)?
Why is accepting that so difficult? Because that IS what they did. And it worked, beautifully. There's no shame in loving that material at all.
But as someone said above, call a spade a spade.