This will become an assailable salient soon and by 2036 we should have dozens of competing phone operating systems and vendors.
We're going to do the same for robotics and everything that gets shipped to us.
What have people done with it? Some mediocre support in libcamera? :) I don't think lack of handholding is an issue here.
Some devices "work" without having real support by having some binary blob or third party code which is written against a specific kernel version. The code has to be updated when the kernel changes, which is why it's supposed to be in the kernel tree, but because it isn't the kernel developers don't know anything about it and aren't maintaining it. So then instead of working indefinitely it works until whenever third party stops updating it for current versions of the kernel. Which is why all the drivers should be in the kernel tree.
Of course, Apple isn't really that interested in supporting Linux since it's a direct competitor to macOS rather than a way to reduce their dependency on Microsoft/Google, so their hardware is more likely than most to be stuck using some reverse engineered alpha driver or proprietary blob extricated from the macOS one.
Not saying pushing mainline is bad. But assumption there's any kind of "support" is wrong. Code is there. That's all. You have to support it if you want it to work.
(This is very particular to HW/drivers support, Linux core is supported well, I guess.)
Also RK3399 is not just pinephone pro, it's way more prominent in other things - notebooks, SBCs, ... whatever. Same ISP is in i.MX SoC, which covers another whole range of devices.
Not sure what 100's of millions shipped units to disinterrested audience achieved?