Should one stick to their expertise/experience area post PhD?
One of the advantages of academia is that, in general, you get to follow your nose into whatever research corner you want to explore. You can change incrementally over time or you can make big jumps. Of course, you probably want to stay employed, so it is requires a bit of finesse and maybe negotiation. 

But the big lesson is that your early work doesn't constrain your mind from think other thoughts and your past doesn't constrain your future if you are willing to work for it. But the same thing is true for people outside academia where such change is possible, though possibly harder. Resource constraint, for example. 

Do what you love if you are able. 

That, actually, is what keeps a lot of people (most?) in academia even though the pay may be less than what they could earn elsewhere.