It looks like Wolfram|One is a subscription product only, while Mathematica can be purchased outright. I have Mathematica now, with Premier Service plus, and as far as I can tell, I get all the functionality of Wolfram|One. The cost of the service contract for Mathematica is less than the annual Subscription for Wolfram|One. (I am looking at the commercial licenses only.)
I have been using Mathematica since version 1.1, before there was a service plan. I hope that the option of Premier Service Plus remains available for the foreseeable future. It provides the services and support that I need.
I can understand that the name "Mathematica" is no longer entirely descriptive of what the software can do. Changing the name may be hard from a marketing viewpoint, but I can see the value of making a change so that the software can escape the 'ghetto' in which it currently resides. As far as I can tell, Wolfram Desktop, Mathematica, and the desktop component of Wolfram|One are essentially the same. It might be useful to have the same name for these apps.
One last thing: the main marketing page for Wolfram|One has an icon showing portable devices (e.g., iPhone and iPad), but there is no reference to these devices in the rest of the marketing pages. Does this refer to the Wolfram Player, currently in beta, or does it refer to a version of Mathematica (by whatever name) that will run natively on an iPad? I remember seeing this demonstrated at a Wolfram Technology Conference. I remember that there were some issues with the first generation iPad, but the newer iPads -- especially the large one -- is much more capable.