Just like how TVs come with default settings that mangle the picture quite a lot, and only some people will fix them.
I'm sure all those things are possible, but it's also pretty likely that they're actually producing and distributing content in 4K and Dolby Vision.
It's just a matter of fact that, for various (many) reasons, people will get 720p streams. Some of these people are paying $3/month for "Premium" with "Ultra HD" and simply do not notice that they are not getting 4K streams. NetFlix would probably rather keep the $3/month, the alternative would be to educate their users about why they are are not getting 4K (can your TV handle it? can your playback device handle it? are they connected correctly? are they configured correctly? do you have sufficient bandwidth? are there problems with your ISP? does your device have the right DRM?) and this is both time-consuming and likely to create a bit of a support nightmare.
But yeah, I think the actual presence of additional dynamic range is pretty noticeable for most well-produced content.
I think this is how perfect society should work when marginal cost is zero.
Netflix cannot directly charge proportionally to your income.
But the cost of your TV set is somewhat proportional to your income.
If you have 4K TV then you are rich, and you can afford to pay more. To allow poor people who cannot even buy FullHD TV to pay less.
So everyone can watch movies, poor folks can afford it, and rich people won't mind the price.
In another word, it would be worse (from a society point of view), if the price was constant, but only rich people could use Netflix.
(BTW this is how people pay for government services with taxes: rich people pay more money (even with flat tax rate) but get the same service).
I've just googled: Netflix yearly revenue is 16B and Netflix spent 12B on content.