Nikon's missed opportunities...

The body of the shutter being made of plastic is OK, the shutter is not subjected to outside stresses.  Also, even though the D4/D800 and D7100 shutters look similar, I believe they are different from one another, they are not advertised as having the same life cycle (although they avoid using the same terms, for the D4 they say "D4’s shutter has been tested on fully assembled cameras for 400,000 cycles", but for the D800, "Rugged magnesium alloy construction Effectively protected from invasive dust, moisture and tested to exceed 200,000 cycles."

I don't think it would be too difficult to have a modern shutter for the D300 successor. I don't think it would be too difficult to build a D300 successor with existing parts. I'll leave the details to the designers.

JC
Some cameras, some lenses, some computers