"Do you think Nikon will continue to make a semi-pro DX body in the future?"
Short answer: Yes.
Longer answer: I believe that in the near term (the next few years, at least) Nikon will continue to support the DX sensor - to include a replacement for the D300/D300s. However, with the pace of development of camera sensors (and the corresponding drop in production costs), I am not so confident in the long-term outcome.
My D200 works as well today as it did when it was new, but today's cameras have significantly improved performance in several areas. Personally, I could not see enough advantages in the D300/D300s for my particular needs. However, I expect the next generation of mid-level (semi-pro?) DX camera will make an upgrade desirable for me. If I am wrong about Nikon's continued support in this area, obviously I will have to re-think my next move.