No - UI-Kit is still going to be available and supported for a couple of Pega Releases - We are just no longer going to make major improvements and changes in UI-Kit to stay in touch with the latest features of the platform. UI-Kit 15 is the latest version of UI-Kit - it was introduced in Pega 8.4 - there will not be any new version of UI-Kit in upcoming releases.

Changes in UI-Kit will mostly be fixes.

No, cosmos wont be replacing UI kit in 8.4 completely. you will have an  option to choose between UI kit or  cosmos while creation of the new application.