Yes, of course it's natural to progress. But progression also implies a gradual process. By essence I don't mean it has to fit within a certain box and remain narrow. That essence evolves with the band over time, it's not something set in stone for the life of the band. The essence of Opeth does not dictate that they must forever be death metal. That's not what I'm trying to say.
Look at Damnation. Not heavy, no growls. But it still had the essence of Opeth in its style and songwriting (at least, to me). For some people the new album will also retain distinctive Opeth qualities, and for some it doesn't.
For some fans heritage has progressed too far for them to like. For some fans it has progressed far enough that it will take some listens to adapt to but will accept and love it, and for some it will still grab them on first listen. It will be different for everyone. I feel that progression should allow the fans to evolve along with the band, even if it takes them some time to adapt.
So I'm really talking about the speed of their progression, rather than defining what their sound "should" be. For me personally, I feel this album is too much of a leap for me to retain interest in them. Others will have no trouble adapting.
I hope that's clearer, because I don't want it to sound like I'm whining because it's not exactly what I want and expect it to be.