I expect more people than just me are interested in reducing tech turnover or using old tech in the hobbyist space, ie. not single handedly. And yeah, as the sibling says, if we're relying on proprietary drivers, we're not really FOSS- my fantasy world doesn't just apply to Apple.

Yes, it would be very hard. But today it is impossible, so very hard is hardly a complaint you can make in comparison.

Did my other two examples seem more reasonable?

I imagine their ideal open source phone OS wouldn't include any proprietary driver blobs, but what do I know...

Perhaps the more generous take is that, with FLOSS software, you don't have to go it alone. My Pinephone Pro will not be running security fixes I write myself. But entirely because of its open nature, it doesn't need to - it will continue to recieve updates for years and years. For once, the hardware of a cell phone is likely to die before the software.

If it gets the support of the right people who know how to write such security software and have an interest in maintaining it, sure. But that's the reason many open source initiatives fall short of proprietary. It's either based on the altruistic whims of a few particular talents, or some company is paying to have them maintain it. And we know the latter is fleeting.

The caveat is that sometimes an initiative can be funded by charity or bounty to keep interest, but relying on generosity for 99.99% of projects is a fool's errand.

No. They want iOS to be open source, because they want to be able to modify a few bits of source code and use their modified OS for their personal use, and then keep pulling updates from upstream forever after that. Like a small fork.