I don't think an iPhone can fully replace a Nikon -- but it is getting close enough that non-professionals have a hard time justifying carrying all that glass around.
The bokeh from my 85mm 1.4G lens cannot be replicated on an iPhone. But the "cream machine" costs more than an iPhone. However, the low-light capabilities of this iPhone shame Nikkor lenses that I spent thousands on.
Bottom line: I sold all my heavy primes about two years ago. I can't imagine a single justification for carrying them around any more. If I was shooting a wedding, or had some other professional gig, then a DSLR is a must. But the consumer and prosumer markets for Nikons is quickly being decimated by the iPhone's capabilities. I don't think in five years, Canon and Nikon will still be on the market. They will be forced to merge (unlikely) or one will go under. There just isn't the demand for the product any longer.