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Exactly.
Apple does this all the time and it's understandable. Newer features are withheld from older devices all the time.

The ability to yellow shift the white point on a lcd screen has been around for more than a decade for the mac. Long before 64 bit.
Keeping a new feature away from older hardware is Apple's right. It's not like they can publicly admit they're deprecating old gear intentionally.

Remember, Phil Schiller saying that there are such and such millions of old PCs in use and that's just sad.

Sad for who? The computer manufacturers. Not us.
I have some older computer equipment, it works wonderfully...for little things. Older in Intel terms is different than older in mobile terms. Any computer with at least a core i7, introduced in 2008 will be usable for basic tasks plus a few more things. Older than than not so much. If all you care about is basic email and web browsing any computer in the last 10 years will be fine for you.
 
If they want to withhold features to entice people to upgrade, so what. The 4 and 4S were supported for years after release. People who purchased the device knew what they were getting and how long they could expect support. It's going on five years, time to upgrade, people!
 
I'm trying to imagine how many people are running out to buy new iOS devices for Night Shift (per the OP's speculation). I'm guessing not many.
Now that's a very interesting question. How many people base their buy decisions regarding phones on what they read in this forum. The more I think on it, the stronger my conclusion is that very few base their buying decision on what they read here.

I believe this forum re-enforces decisions already made based on past experiences, past habits, and general correct perceptions as well as misperceptions and bias opinions already formulated. I did an unscientific pole regarding if anyone had changed their phone based on what they read in this forum a while back. Had near 1,000 views with two people possibly saying they changed phones.
 
I didn't rush out to buy a 6+ for night shift. But dang! It's really great at night. That harsh blue tinted light always bugged my eyes. I slept fine either way. But after using night shift at night, the thought of staring at a bluish screen blows.
 
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