Kempe has upped his game for certain. I do think he's still too streaky to be a top-line forward. And by streaky, I don't just mean his production. I mean his engagement and effort into the forecheck.
I don't think "expendable" would be the best word - but the Kings have a lot of forwards and a hole in goaltending and left defense. It makes sense to trade from a position of surplus to fill this hole.
With Kempe, he has a few issues surrounding him:
Fiala is more skilled.
Kaliyev is producing at a better clip at the same age as Kempe was. With the Kings needing to build a younger core, it doesn't make sense to trade him (if the Kings already had a new core in, trading Kaliyev would be more defensible).
Viktor Arvidsson is more consistent in his execution
Danault, Lizotte, Iafallo, and Moore fill more niche roles.
So, it's not so much about "expendable" but more about trying to fill holes. Kempe provides the best value to fill a quality player, and the Kings would be hurt the least (long term and short term) to trade Kempe. Plus, they need to have cap space for all these contracts.