Okay. I see what you're thinking. Unfortunately, it's not quite right.
Expect a few WP8.1 apps to be removed from the store, but expect most to remain the way they are now. Some WP8.1 apps, particularly those from larger companies, will continue to receive updates for some time, just don't expect them to be big updates with a lot of new features (you'll receive most of those updates only to keep them compatible with those company's evolving backend services). Those companies might also release a UWP app alongside their existing WP8.1 app (Plex is an one such example).
However, don't expect anybody to develop new apps for WP8.1. Ever. Pretty much all new apps targeting Microsoft's mobile OS will be built exclusively for the UWP.