Metallica didn't go mainstream, the mainstream went to them. Sad But True, Of Wolf and Man, The God That Failed, etc. are some of the heaviest songs they've recorded. Plus Bob Rock made that album sound great. The production is
way better than the production on Justice. The Black Album sounds heavy as shit and it is. Shorter songs don't equal less heavy.
There was plenty of super-catchy hair 'metal' that was heavy. Including the song/album that made Metallica hire Bob Rock. Dr. Feelgood. Warrant and Cinderella had some fairly heavy stuff. That doesn't mean it wasn't also drivel.
I'm not drawing a direct line between the music on the Black Album with that other garbage - the point is - "heavy" or not, it wouldn't matter. The problems were outside of the fact that Metallica still used distorted guitars. You're almost arguing that "James Hetfield still growled on the album!" So what?
As for the production, I hear you but I don't really give a shit. If slick produced records translated to better records, then Def Leppard with Mutt Lange would be better than the Beatles. Production is just presentation, and I give much more of a shit about the songs. Every single track on Justice is better than everything on the Black Album. That's just my opinion. Generally, I just like progressive hard rock better than I like poppy hard rock.
But those aren't the problems I am talking about anyway. The problems were the change in the songs and I'm not just talking about length. Do I really need to explain the differences? I don't even think I own the Black Album anymore but I could attempt to explain.
All that said, if you like the Black Album, more power to you. But saying that the mainstream went to Metallica is simply at odds with reality. Name one successful band that pulled a 'Metallica' and I'll name five or ten that tried the same shit and failed because the audience wasn't there. The mainstream went to Pearl Jam and Nirvana and the Smashing Pumpkins, etc. because that's where all the major labels went to go make their bundles of cash by signing the latest post-grunge and alternative rock derivations. They weren't beating on Anthrax's door hoping they could deliver a Black Album of their own. Because Anthrax had already hired a different singer and tried and failed. And they weren't the only ones.
But anyway, where were all the mini-Metallica's in the 90's? After all, the mainstream came to them. The only lifeline that metal had in that whole period was a bunch of nu-metal bands in the late 90's that associated more with the groove metal of Pantera and Sepultura.
Also let's not forget that Metallica tried their best by '96 to seem 'alternative'. Remember that shitty video for Until It Sleeps? I do. They cut off all their hair and tried the best they could. Even somehow getting a slot on Lollapalooza. Another example of the mainstream coming to them? Yeah right. By that time, they had already come 'out of the closet' as commercial opportunists. And if it were another band, fair enough. Whatever. But Metallica were the torch-bearer for anti-bullshit in hard rock. And they were about all there was at that time. And they didn't evolve by hiring Al Jourgensen's producer and trying to mimic closer to Ministry's industrial sound or look into what Trent Reznor was doing or even to Faith No More's 'alternative metal'. They went for the sound that they knew would sell. And yet, according to their own words all throughout the 80's, they hated that sound. So why else would they hire Bob Rock? If you can explain that then you can change my mind.