Great points, and mostly agree.
1. as far as I know, the jack takes up quite a bit of volume, it's a long and empty space, that could be used for battery, heatsink etc. It's a minor save, but smartphones are some of the most volume-dependent devices, and marginal benefits can be high. i.e. most users may swing around say 4 hours of screen on-time, and get frustrated with 3.8h of capacity. Small increases can push them over the contentness-edge. I'm very much reaching here, it's not a very compelling argument imo, but there's some benefit there nonetheless.
2. So apparently I'm the edge case who never charges while listening, it's just not a thing for me. I charge once, at night, and it's fine. (Galaxy S7). In a car I'm happy with my phone's speakers or a wireless setup, and in newer cars especially a usb-audio or bluetooth audio setup shouldn't be a problem. (ignoring obvious transition issues). And especially here, in a private, safe, storage area like a car is, I'm okay (obviously not thrilled) to buy and one-time mount a $25 dongle. Not the end of the world for me. Same with my fixed office space, and that's about all big usecases I can think of.
3. Completely agree, however it must be noted that stuff like this pushes wireless audio quality forward. There's suddenly a forced market demand when the port drops, which opens up industry incentives to push wireless price/performance ratio improvements which I feel are around the corner (and partially announced already). And tech wise a lot more is possible than the quality we've had in the past years. And price wise, a LOT more should be possible than the past few years of price points. Also I'm expecting Apple (let's not forget, a producer of custom earbuds for years, and buyer of Beats) to drop decent wireless tech, too. I want good wireless audio tech regardless of whether I'm forced to use it lacking a port or not. (!) Again, obvious transition issues but it's alright.
4. Rumours are hey're looking to reinvent their base model with oled screens, no home button, edge to edge screen, delayed to 2017. i.e. a move away from a phone that has ports and buttons, towards a device that's completely digital, mostly screen and almost fully wireless. And I think that makes a lot of sense, stylistically too, if rumours are true and they can pull it off. That can reinvigorate sales, because let's be fair, the iPhone 6 was 'good enough', kind of like the Nexus 5 (still a lovely phone I'd happily use) was for the mid-section, the 6 was at the top-end of the market. I've got lots of friends with a 6 who just don't care for iterations. Something radical is needed to get them to buy, and dropping this port long-term as part of the 10y iPhone anniversary update is part of that narrative. Lots of speculation here of course, just thinking out loud to try and discuss your questions.