What "IF" no D300 replacement? What next?

Totally agree!! You told the importaint thing: 16-85 VR is better than 24-120 in center->edge sharpness difference. Now I am looking at my 60 x 40 cm print from D300 and I ask: "Can I really enjoy better, FX quality?" I must admit: NO.  I want D400 upgrade as the new toy, which sometimes will make better image quality and better capabilities and only I willl see the differences, my family, my friends not, of course.

I can ask me: how much very large prints do I make from ISO 100-400 and how much very large prints do I make from ISO 800-6400?

ISO 100-400: 100% large prints

ISO 800-6400: 0% large prints

What is my largest print  from ISO 1600? It is A4. Can I see the worse D300 ISO 1600 quality on A4 print? Only by studying from very near distance, maybe. Is it such big problem to sell DX and buy FX body and FX lenses? No, in any case.

Would be pleasant to buy new D400  and try everything what is new, enjoy 100% resolution of ISO 3200 indoor low-light photos? Yes, of course. And if I will not enjoy the new toy, will D300 limit the beauty of my photos? Of course not.