On the contrary, I think Nikon will continue to release new DSLRs and lenses. The intention is to enter a new market and keep the old, developing each technology and their strengths.
That, of course, depends on the behavior of the buyers. Interestingly, the rear panel of the new D3500 looks very similar to that of Z bodies. The buttons on the left side of the screen are totally gone, even though the screen is fixed.
Apparently, D3500 has the double function as entry model both for the DSLR and Z.