To me it kinda felt like the old Opeth-sound had run its course and was starting to get stale. Don't get me wrong, Ghost Reveries was amazing, but that was 9 years ago. I feel like they tried to do a genuine continuation of the sound on Watershed, and sadly it fell a bit flat. A good album but not a masterpiece. So by the time Heritage was due to come out, I think they had milked the old sound as much as they could.
With that said, I do miss the heavy element of their music. I don't necessarily miss the growl, I think some songs on Watershed, like Porcelain Heart, pulled off the heavy without being growl-heavy. The interplay between heavy and soft was so amazing, and while I can understand if Mikael doesn't want to growl, it's a bit of a shame that the metal element will be lost completely. Even if they continue writing 70s retro prog sounding songs, a few heavy sections here and there where the guitars sound heavy, evil and doomish, that would add a lot, even if the mainframe of the song isn't heavy.