Your argument regarding 5s owners don’t not hold water.
I use a iPhone 5s and the speed deference in daily usage compared to the SE is not significant, and support of Apple Pay is irrelevant as many countries does not have it yet...
In Denmark we just got it a few weeks ago, but we have a banking sector which have been good at making mobile payment solutions, leaving Apple Pay without much influence. Had they released Apple Pay a year after its US introduction they would have a good market share as iPhone are the most used phone in Denmark.
And as you have properly read on the forum, there is a substantial amount of users which have upgrade to the size of the 6 & 6+ design who would like to go down to a smaller phone but do not want to give up functions.
We can’t have it all...
So if they made an iPhone SE whith A11, 3D Touch (still not in the SE), water resistance, better camera, and so on... along with the marketing the new models receive, we would see a difference in how the markets share holds up.
It is not like you would ever compare Fords Model T with a modern car