I can see the 6 and 6 Plus being dropped as they have 1GB of RAM and is almost 5 years old, but having the SE being dropped would suck.
This is but one way to force users to upgrade; via software compatibility. That said I doubt the SE will be pushed out.
Reasons "SE" will get iOS 13 support:
Current production
last months 3 fire sales
India
Apple will have to do MUCH better - either with pricing yet most likely with unique hardware to get users to upgrade; a very compelling reason:
iPhone 4s - Siri
iPhone 5s - TouchID
iPhone 6s - Speed, 3D Touch.
iPhone 7 - IP67 Water Resistance, Better Camera, Faster CPU/GPU, sweet Black Colour options
iPhone 8 - Bleh (Qi Charging was NOT enough)
iPhone X - FaceID, Wider Screen and OLED, much faster hardware and better cameras than the Plus!!
Next year, 2020 something BIG will hit.