Fairphone 6, recent with replaceable battery: 9.6 mm
Galaxy S5, has a replaceable battery, released _12 years ago_ - battery tech has improved a lot since then: 8.1 mm
iPhone 17 Pro Max: 8.8 mm
iPhone 12 Pro Max: 7.4 mm
We want to make phones as thin as possible so the latest flagship iPhone is 1.4 mm thicker than the one from 5 years ago? A whole 0.8 mm thinner than a recent one with a replaceable battery with maybe 0.1% of the iPhone's R&D budget, and 0.8 mm thicker than one with a replaceable battery made 12 years ago?
The bootloader was unlocked in many regions (and became unlockable in all regions). Custom roms were abundant.
And it was waterproof.
(In the subsequent decade+, I have heard it said over and over again that this is an impossible combination of traits. And yet, there was a time when we had all that.)
Rugged phones with removable batteries has vastly superior IP ratings. Glues go bad faster than O-rings used in removable batteries do.
I've had water intrusion with an iPhone, and it drove a sales of a new display panel from myself. Not so much with an actual rugged phone.