Food should be free, because you shouldn’t owe farmers, distributors and retailers money to make the food available to you.
But i get, why it might feel stingy. To each their own.
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).
Although I'm so far on the other end of the spectrum it's not even something that came to mind in my initial criticism. I'm the programmer that mostly works on old/outdated legacy systems that will never look "good" to anyone, and even then I rarely deal with the front-facing UI, usually spend most of my time working on low-level and background stuff that the user never sees. So I didn't even think about the relationship between light/dark when it comes down to unifying the two experiences with a "core" experience. If i were working on something like that I would want to change as little as possible between the two, though in that design consideration, then every element added to the program would have to be "compatible" with the core design, and the user's experience in both would have to be similar. So with that consideration, I could see why it would not be simple to implement/maintain.