I don't understand why people ever doubted Immortal without Abbath. Demonaz is the one that gave us Pure Holocaust riffs.
For me it was a combination of things. The biggest being I just wasn't sure he'd be able to physically play. I didn't realize he had had surgery until I read an interview within the last few weeks. Not sure if it was common knowledge before that, but I certainly had no idea.
I think at the time of the split Abbath said he was going to do a new album, as did Demonaz and Horgh. Iirc, they said they were going to do it themselves and not replace Abbath. Within like a year (I think) Abbath had dropped his album and started touring and Immortal had been pretty quiet. That was largely due to them having to write entirely new material and them just wanting to buckle down and get shit done. But at the time, I wasn't sure they were actually serious. Then they released the tracklist and I was like, "I'm pretty sure some of these song titles are reused." Side note, Mighty Ravendark is one of the best tracks on the album and maybe one of my favorite Immortal songs ever, so shows what I know.
Once they released that first single though, I realized I was being pretty foolish having doubts. It was aggressive and full of energy and purely Immortal. I feel bad for having doubts as well, since when you compare Immortal's new work with Abbath's it seems pretty clear that the soul of Immortal is with Demonaz. Abbath's record sounds like what would come after All Shall Fall, which is fine, but it's moving more away from the kind of black metal I like. This new Immortal album though, is this wonderful return to form. It captures the soul of their music so perfectly that I almost can't believe it's a new album.
So yeah, I had doubts and I feel bad for it. Those guys are clearly still extremely passionate and they made an incredible album. I hope decide to tour and I'm lucky enough to see them play.