It is a fact that both bands greatly increased their fanbases by changing to more accessible sounds.
I would say that is not completely true. I don't think 'Epic' as a very accesible album as any Metallica has released (Comparing a thrash/heavy/rock band with a Black/"folk"/progressive band has very few points to compare, doesnt?).
If we must to compare albums, TAC is so much more accesible and it's the less agressive Borky album IMO, since is has no blastbeats and is sung 80% with clean vocals, also I don't sense any 'darkness', nor 'evil' on it, so why TAC must be superior to Emp or Epic? (for Simen's clean vocals?).
Now with Vintersorg is more like 50-50% on the clean/grim vocals, the blastbeasts and double bass patters are somehow present on almost all songs with an overall fair speed (Epic is so much faster than Empiricism, TAC and even TOD, in fact it's like the Q level on terms of speed IMO) then where's the problem? The progressive path taken my the band is the reason of the lack of 'brutality'? could be the massive vocal harmonies done by Mr V and Lars the guilty? the crystal clean production? the less 'br00tal' grim vocal provided by Mr.V? maybe all of then, but what Borknagar is today is far better IMO than the most of plain 'dark/evil/tr00-kvlt' black metal around.
You're right. This band could write Si Monumentum Requires, Circumspice and Vintersorg would still make it sound like a Disney cartoon.
Personally I think the Mr.V's vocals fit well with the actual Borknagar sound and songwriting. I enjoy the actual mix between the black and the progressive music, and neither Garm nor Simen could do a better job on the actual songs. What I like from Vintersorg's grim vocals is they are agressive enough to fit a black metal song on Borknagar, but are not so 'evil' nor br00tal to overwhelm the rest of the band or to give the wrong impression about the music, he has IMO an 'avantgarde' vocal tune, not a 'black metal' voice (Cornelius's latest vocal offerings comes to my mind with this sentence); also the lyric theme on Borknagar is very different than the rest of the black metal acts, so I don't see any reason to sound evil nor dark when the whole band concept doesn't fit with those adjectives. About his clean vocals, I think all of us (or most of us) are agreed he's absolutely great.
Even when I know it's just an example, I say Si Monumentum is boring IMO as the most DSO catalogue. It's not like I don't like that kind of music, just I don't like them or they has nothing who interests me a lot, the same goes with Limbonic Art. I just like some songs but I don't see myself listening an entire album. I prefer to hear both Sirius Albums instead LA. That's the good point of tastes...no one is right and at the same time all we are.
Metallica's intentions were to sell out, and Borknagar's were to expand musically. These motives are different, but the outcomes, in terms of quantifiable popularity, were the same.
So what? that's bad? I don't think Oystein has that in mind. If the band wanted to earn money, they could do endless tours like Dimmon Burger.
Could be the music quality and the impressive line-up (at least until when Asgeir was on the band...now without Asgeir the line up seems to be weaker but I think Dave will do a good work) one of the reasons that Borknagar is more popular now??