> the phones got thinner
No. They didn't.
iPhone 5: 7.6 mm thin, 6: 7 mm
iPhone 7: 7.1 mm
> While Apple could likely have gotten their water resistance even with the headphone jack, they couldn’t have made the phones as thin.
Except that's a pretty obvious lie. Not just that the phones did not get thinner (or lighter), they stayed around the same thickness (+- 0.5 mm), while getting larger, much heavier and much more expensive. But also the thinnest Android phone with a 3.5 mm jack is just 5.1 mm thick, for example. Sony even made a waterproof phone that's 6.5 mm thin and still has a 3.5 mm jack, which is thinner than any iPhone ever.
Everything about this argumentation is wrong or an outright lie. The only reason they did this is because they could moneygrab through accessoires better when they eliminate standardized I/O.