I have glasses, they are expensive, I understand that. I have sensitive eyes and overtime they fatigue much quicker then they should.
No one owes you anything, especially at cost. However when it comes to a dark mode option, it doesn't cost the developer anything, it's a theme change.
The other options are great enough for a sub that it's worth it with or without dark mode; the mobile editor and cloud storage are good enough reason. I only felt it was even acceptable to state dark mode should be free because for a good portion of people it's necessity and there are other good reasons to buy a sub. I usually won't test a free application unless it has dark mode by default because it's that hard for me and I know from others that they experience this as well.
With or without dark mode, if you use something like this daily, you will end up caving and paying. Without dark mode, there are people like me who can't even test it out properly (it's physically painful to look at a white screen for too long).