But as such they have tweeked Denim for every device and stripped off major stuff like Lumia Camera 5 with excuse of hardware limitation. How much resourses would it take just for such small thing?
Actually, they have not tweaked Denim for every device.
For many older models its little more than a name change, or perhaps a minor fixing-up since Lumia Cyan firmware works fine as is, given the relatively minor nature of the Windows Phone 8.1 Update 1 update its corresponding with.
Hmm, maybe I can make an analogy. You're assuming they put the effort in building a skyscraper for each and every phone in the beginning, and then putting the effort in to demolish large sections at the top because "hardware limitations" and "evil-ness."
They don't, maybe they add another floor or two for older devices, but there isn't going to be much return-on-investment in sinking even more resources on already-superseded devices.
It takes effort to do even seemingly minor features. Perhaps they decided it was better to focus on newer devices and other things in development than investing in a brightness profile option many may not realize is there.
Now of course, this is all conjecture, I could very well be wrong since I don't work for Microsoft, but if my Computer Science class a couple years ago means anything, coding can be a pain.