I'm not sure I agree. Functional Reactive Programming is a declarative way of doing user interaction, but it's still a computer science research topic (Elm used to claim to be FRP, but has now decided to move away from its old allegedly FRP-like style). I need to play around with the latest FRP research libraries some time!
In addition there is also machines/pipes/conduits, which all mix imperative I/O with declarative style code in an interesting and new way.
FRP is awesome and is an example of how something previously thought of as an inherently imperative problem can be much easily done declaratively; we completely agree on this!
What I was talking about is: almost all lower-level abstractions are designed with the view of computer as a state machine. If we are solving a problem at an abstraction low enough to make us deal with this analogy, it is easier not to have a declarative layer in between.
machines/pipes/conduits are all very interesting and I was going to mention them too, but I still don't think they solve this problem in the way OP was referring to.
In addition there is also machines/pipes/conduits, which all mix imperative I/O with declarative style code in an interesting and new way.