Nikon already has a good range of DX cameras, albethey DSLRs with mirrors, so they have that format fairly well covered already.
As to the "mirrorless" line, since there isn't any real advantage other than eliminating mirror slap, I don't think Nikon will be losing significant market share even if they never produce a mirrorless camera, in either FX or DX size.
What real advantages are inherent in mirrorless cameras, and why do people want them?
(edit) O.K., I just thought of one; silent shooting.
At one time SONY didn't make Mirrorless cameras. They had their DSLR cameras.
Based on your statement that "I don't think Nikon will be losing significant market share even if they never produce a mirrorless camera, in either FX or DX size."
Technically then, SONY should not have sold one Mirrorless camera against their DSLRs.
And Nikon and Canon should never lose market share against Mirrorless cameras even in their own lineup, and neither should produce any at all.
But what is another technical advantage of mirorrless? FPS
Nikon has their 20fps Nikon1 (others are 15 and 10fps). They also had their now gone from their website "pro" mirrorless with a fixed lens that also did 20fps I believe.
But it's going to cost a lot more to get a DSLR's speed up to 20fps, and make it wholly affordable in the under $1,000 camera kit arena.
As SONY sells more mirrorless (and FujiFilm, Canon, etc) , you wonder what market share Nikon could have had additional to their DSLRs if they just do it right versus just making a weak alternative that they've hopefully had up until now.
Video has also driven the market. You personally, may not do video, but that doesn't mean other people don't.
One's individual Personal preference doesn't drive overall market share.
The camera makers have to move with where technology is in order to continue to compete.
You can probably list of a bunch of tech companies that thought newer tech stuff was fads and decided to compete too late and their names have gone to history.