I don't think OKC wants Kemba at his extension numbers and he'd be a weird fit next to Westbrook. Charlotte likes Kemba because he's tied up and willing to be there, something you can't say about Rondo or Green if Green opts out.
Rondo and Green's contract situations lower their value. For small market teams, having players who want to be there and are locked up contract-wise is more important than overall talent.
Finally I don't think the Thunder would see Kemba over Jackson as a significant enough upgrade. They might even see it as a downgrade in terms of fit and contract situation.