It's almost a stretch of the imagination to think back and remember that I originally came to UM because of Opeth and their official forum. They were my favourite band for most of the 2000s.
Where they've been headed ever since Watershed is really uninteresting to me though, and I've barely spun any Opeth records in years now. Heritage wasn't so much a shock stylistically, as we all knew Mikael was heavily influenced by prog, it was mostly a shock in the sense that the quality of songwriting was so poor compared to their prior, seminal records. The record just diddles on ceaselessly. I understand the desire to play prog, but Heritage is re-treading ground that was done better by bands over 40 years ago. It just makes no sense to me. Above all, I can't understand why it was released under the 'Opeth' moniker. It could just as easily have been a side project. If Mikael is truly over the growls, over the metal etc. then maybe it's time to call it an end to Opeth and pursue his tastes through other avenues.
Needless to say, I'm not really piqued for Opeth's upcoming record in the least. It doesn't really matter though, because looking back through their body of work there is so much quality that it rounds out a great career. They've had a much better track record than most bands which last for 20+ years. If you feel the same way I do you can just bust out MAYH or Still Life every couple of months and marvel at how creative and inspirational metal music actually was at one point. Those records really just get better with age.
I agree their old albums do get better with age, but I disagree with the rest of your post. To say they should've released it with another name is just pigeon-holeing bands into a certain style and that if they do something different they should just call it quits and make a new band. That's just not practical on all ends. If the band dies, you will never get the chance to hear those classic songs live, even if in many cases with the new "touch" of the current band. Also, telling a songwriter/band leader what he should do with HIS project is also a bit insulting (I'm not saying you are insulting Opeth or Mikael, I hope you get my point with this) because he has no reason to give up the moniker that spelled succes for him/them and start over, with short setlists, smaller attendances, etc.
Also, it's a very personal opinion, but most of my favorite bands are bands that have changed over the years to a point where they easily could've changed name more than once in their career. Opeth, Paradise Lost, Anathema, Katatonia, Mayhem, Carcass, and many others are prime examples of this. I love when bands do different and unexpected things, I end up getting bored of bands doing the same thing every album even if they were favorites at one point, like has happened to me with once favorites Nile, Cannibal Corpse, and other (mostly extreme metal) bands, which I still very much enjoy, but can't really spin as much as I used to.
And Heritage IMO is awesome. To say that it's something many bands did better 40 years ago I think is not quite true, of course there's that huge influence from those bands, but there's also the strong Mikael Akerfeldt trademark in the songwriting, voice, lead and rhythm guitar playing that can be traced back to their older albums but now in a completely different environment, it's what I love about those bands, like Paradise Lost. When playing Doom/death, electro pop, goth rock or Hard hitting Gothic metal you always had the trademark songwriting and lead playing of Greg Mackintosh but in different environments, along with the lyrics and vocal melodies and harmonies of Nick in all the different "eras" of the band. And you can't forget in Heritage you also have the immensely powerful input of all the other members, be it Axe, Mendez or Fredrik. I think Per, as much as I admire his playing in previous albums, he was obviously under the strict control of Mikael for this one, specially cause everyone already knew he was leaving, so he pretty much went with the flow on this one, not to say it isn't still an outstanding performance, cause it is, but I feel he put in much more of his trademark sound and execution in Ghost Reveries and Watershed than in this one. I could be wrong of course, it's just how I sense it.
That was a long (not really a) rant.