Most people using Atom or any other open source project, would likely have based their decision at least partly on the organization backing the project, especially in a professional setting.
For the most part, when a different organization picks up the development of the project (if one does) it's a completely new project at this point (there are exceptions, such as a backing organization continuing to back the project, but deciding that it's better supported by an independent trust, and therefore building a transition plan for that).
The best example of this is looking at any Apache supported project today. There are very few projects that were handed over to the Apache foundation that people would want to continue using in their post Apache handover date, even if they were amongst the lucky few that still have active development (OpenOffice, for example).
Sunsetting is absolutely the right term/approach to use.
For the most part, when a different organization picks up the development of the project (if one does) it's a completely new project at this point (there are exceptions, such as a backing organization continuing to back the project, but deciding that it's better supported by an independent trust, and therefore building a transition plan for that).
The best example of this is looking at any Apache supported project today. There are very few projects that were handed over to the Apache foundation that people would want to continue using in their post Apache handover date, even if they were amongst the lucky few that still have active development (OpenOffice, for example).
Sunsetting is absolutely the right term/approach to use.