Why does Samsung not keep producing the Galaxy Note 4 and Galaxy Note 3?
These devices, despite of their release date as flaghsip device in 2014 and 2013 still perform very well, and are solid mid-class phones today.
A great example: Note 3 (2013) versus A7 2018 (2018):
The Galaxy Note 3 is equipped with:
Air View hovering controls and tooltips
Air Gesture controls
Humidity sensor (hygrometer).
Temperature sensor (thermometer).
2160p video recording (yes, the Galaxy A7 2018 still only has 1080p at 30fps‼)
1080p@60fps smooth video caputuring capabilities,
Slightly larger battery
User-replaceable battery (thus custom back covers and wireless charging cover)
S-Pen, the highly functional stylus.
Multi-window pop-up windows with small minimum window size.
Dark user interface (as known from legacy TouchWiz versions) with more icons in addition to the text, also in context menus.
More customizeable and ultimately vibrant camera application user interface.
Homescreen with more options, page slider and more drag/drop shortcuts such as AppInfo.
All of which the A7 2018 has nothing.
Of course, the Galaxy A7 2018 has some advantages such as quick-launching camera, but why does Samsung not keep producing previous flagships that could be an excellent mid-range device today?
And features such as the camera quick launcher can simply be implemented via software updates that hopefully do not have side effects such as the uglified Lollipop user interface after updating S4, Note 3 and S5.
There is no mid-range Galaxy Note (there is LG G Stylo however), except Note 3 Neo, which got sold much longer than original Galaxy Note 3 despite being not as good as the original Note 3.