[chesterbeatdeath]

Now I await the reasons I'm wrong to want a non-Android Linux phone over an Apple product.

Alright, I'll take a try at this

1. It won't take off.  Sailfish tried, and is failing.  Mozilla tried, and failed.  Ubuntu tried, and failed.  PostmarketOS is cool in that it's not trying to be a mainline OS, so it has it's niche.
2. The Linux kernel isn't great, but then again, XNU isn't amazing either
3. iOS/Darwin has good security defaults, Linux doesn't.  They'll have to modify it a lot to get it to the same level, and I doubt they will even try

I love talking OSs with you; I always learn something.
I think people need to keep trying until someone gets the winning formula, as far as the Librem failing. No one made a powered flight until they did, que no? I think the security aspect will be helped along by the physical kill switches of the phone itself.
Without eyes and ears there's a lot less for the phone to watch. And endgame security is essentially up to the user after a certain point. I keep an eye on my system processes all the time and have researched which ones should be there, and even the ones you can be paranoid about and still kill without the Kernel crashing. This is what any responsible Windows user should be doing as well. With a system process manager installed with the shipped OS, one can keep an eye on what's running or if anything is that shouldn't.
iOS is pretty pimped out and locked down when it comes to out of box security. Apple also has an in house dev team working constantly to work out kinks, no current distribution outside maybe RedHat has that level of development going on (and let's face it definitely not with the millions behind them Apple has to play with).
There are plenty of other, better alternatives out there to what Librem offers, but I say support their trying just in case they are the ones who get it right or even point future endeavors in the right way.
In the end, the Librem and the software that may come out for it are for specialist users, the likes of us, who give a shit about security. I'm fine with everyone else playing happily on their completely open handsets.
So yeah, you're right on that too, it's a niche market. I happen to be in that niche so I think it's super cool.
Stay hydrated.