OK, normally you're pretty clued up as far as music is concerned, but to say there has been no innovation in rock music is ridiculous. Sure, all rock bands have their influences, that's just because rock greats can truly inspire some people. Having influences doesn't mean not being innovative. Obviously I'm no expert on pop music, but all music takes inspiration and influences from somewhere, whether directly or not.
If we're talking about innovative rock albums of 2015, I can name a good few off the top of my head, though it'll be limited to my own personal taste, which is mostly the progressive realm:
Atmospheres - The Departure
Ghost - Meliora
Leprous - The Congregation
Arcturus - Arcurian
Subterranean Masquerade - The Great Bazaar
When you say a genre isn't innovative, you're probably only focusing on the more mass-market artists in the genre. I'd argue in most genres the mass-market big names are on average less innovative. I'll forgive you for perhaps not having heard of most innovative bands in the rock/metal scene, but you can't make generalisations just based on what you've heard if your knowledge isn't comprehensive.
I'm definitely not arguing that there's no
innovation in rock (and I was definitely being hyperbolic about not a single innovative album in 2015, but more on that later). I'm arguing that there's less innovation in rock than in pop, which is in general true, by the history associated with rock. Influences are fine, and I think most innovation comes from reinterpreting influences to your own music (hence why I listed all those genres rock are occasionally sourcing from). Lots of bands do just sound like they've come straight out of a certain decade though. And that's certainly not a bad thing. Great albums come out of it.
It would be obviously be impractical for me to scour every album of last year, although I'd like to listen to as many as possible. The gamut of music I took in though makes me feel like I can make a fairly accurate assessment on the state of music. Of course, I'll misjudge from time to time though.
Taking the Ghost album, Meliora, since that's the one your list I did listen to last year, I disagree with you. I think, in general, it's not that innovative an album. It doesn't really tread anything new for hard rock and metal at all. I think it's quite safe. I don't hear anything immediately innovative about it, besides perhaps the occasional organ gimmick. On the other hand, the arrangements are surprisingly clean, and the riffs and melodies are almost exclusively great. Strong cohesive album. But I wouldn't say innovative.
I mean, it's a bit naughty of me to be using innovative as a black-and-white qualifier. It's a sliding scale of course. That's also important to note. And I don't think innovation is that
important a measure. Sure, innovation can make a good record more enjoyable, especially as you can immediately identify it with that artist, and engage with it on a more conscious level because you're listening to something new, but a lack of innovation is not a bad thing in the slightest. Hell, my favourite rock album last year was probably that Baroness album, Purple. That felt like quite a derivative-sounding album, but was just solid all the way through. It was innovative in the sense that the jazz-inspired bass was interesting, but that was subtle. All of these good albums are innovative in their own ways, and that doesn't have to be an upheaval in sound. It could be, you know, using a weird instrument, or cleaner arrangements than historically in that genre, or whatever.
I've not really heard a rock album that's literally felt like something completely new in the past few years like FKA twigs' debut, or James Blake self-titled that basically invented a subgenre.
In general, not important though. Good rock music is still being made. Good pop music is still being made. Innovation shouldn't be a primary argument. The only reason I bring it up is because OP is using it as such.