The dpi value does not affect the image - it is contained in some files and is used for printing the image - to tell the print driver how big to make the pixels and hence the physical image.
In Photoshop 100% zoom means 1 image pixel maps onto 1 screen pixel. The dpi value in the image file does not matter, nor does the physical size. Only the number of pixels.
To see an onscreen preview of the physical size then your screen monitor resolution needs to be set accurately in Preferences > Units & Rulers. Then you can use View>Print Size
but, Why CC 2018 opens my CC2017 files to 300dpi, if they were originally 72dpi?
Messing with Unit and Rulers didn't help — Retina screen is 144dpi, so files that are 72dpi are shown at 50% no matter what.
On Mac, I fixed this problem by going to application file and checking "Open in low resolution" in "Get info" popup. Seems to be the problem every time Photoshop is updated — it doesn't remember this preference.
I just tested this. I opened three different files, one at 72ppi one @ 200 ppi and one @ 350ppi. I opened them each once in PS 2017 and then closed PS 2017 and opened them again in PS 2018. No difference. I couldn't reproduce it.
I will continue working the cc2018. Probably just adaptation. Thank you all for taking the time to respond.
I am having this issue as well. With 2017 I could see a size difference with the changes in resolution. Now, everything I export appears to be the same size (at 300dpi) no matter what. I wish I could get rid of 2018 and go back to 2017. I have experienced many bugs in the few days I have had it.