But that's the point. We don't know what actually gets carried through to the next expansion, and what could change with a massive revamp.
Did you foresee Metamorphosis being taken away at the end of WoD? Did we all know that DK's were going to lose two tanking specs? We know that Unholy would be revamped to be a Pet-centric spec instead of DoT-centric in Legion? There's a lot of factors involved, and like you said, even the unlikely scenario where a DK gets its pet spec gutted is still a possibility.
Nothing is permanent, and that has to be stressed.
The last time we got a new class, we had a pretty massive all-class revamp. Prior to that, we had Mists of Pandaria where a new class was introduced and we had a major shift in the Talent system. I mean, suffice to say, with people already getting tired of Borrowed Power mechanics and crying out for something more substantial, it's possible that a new class addition will come with another major shift to all-classes. And frankly, we're at a point where it doesn't even need to be a *new class* for that to happen, since there are so many other alternatives as well such as 4th spec or Class Skins to consider what is actually possible.