How can you have the same city have a different distance to the same fixed point? Cities don't move...
It is giving more, as you've shown. Just not more total profit/time. Size of the city can also play a factor...
"How can you have the same city have a different distance to the same fixed point? Cities don't move..."
Tested in WB, builded a trader in my city washington, then moved the unit in WB into capital city of the viking 366 gold, then moved the unit in WB to delhi capital india, better city then the vikings and half arround the globe and only 390 gold. Long distance no win.

But you also can test it building two trader units in different citys, best alot of different distances in game (Trader unit needs to be build in game not just placed in WB to get the home city for trade calculation) then go to WB and move the two units you build in differnt distanced citys via unit select to the same foreign city and you have the same city calculation with different distances.

Move unit builded in game in WB then exit WB mouse over conduct trade mission to look revenue.
"It is giving more, as you've shown. Just not more total profit/time. Size of the city can also play a factor..."
Distance is completly not profitable now, respawning (Conducting a trade mission to respawn) gives +100% faster currently.
Like: City 10 plots away = 100 gold + respawning trader unit = again 100 gold in 10 turns
Like: City 30 plots away = 110 gold + respawning trader unit = again 110 gold in 30 turns
So only guys with no trading knowledge will go 30 plots when they can go 10 plots for more.
So a plot distance should give about 10% + more per plot not 1%? Then distance should be profitable for 10 plots more to go
(If you have a better or same good city more far away). But it will also increase the revenue for the first 10 plots moved. Hmm have to think about, maybe +10% per plot is to less. but on a base of 200 it will be + 20 gold per plot, sounds better then + 2 gold per plot. Has to overcome the respawning of the trader unit.
Best will be maybe a increasing +% on distance like 5 plots distance = +1% per plot, 10 plots distance = +10% per plot, 20 plots distance = +50% per plot, more far distance = more revenue. (If the city is same good as the closer city) If that increasing of distance revenue can be done. Upscaling with distance, not per plot +1%.
Then you will think about new strategys to establish far away trade routes and how you can do that and you will maybe take the risk for a long dangerous distance. Good for a strategy game. And you have to do a good trade on deity to survive.