Smartphones are fragile without a case. They should have one, and maybe manufacturers should make that clearer, but a world where removable cases didn't exist would just mean that the case you get is the one that the manufacturer chooses for you and permanently attaches to the smartphone. Less options for you.

Just get a case.

I am also more than willing to carry a slightly thicker device if it means greater durability and easier repairability.

Me too. It's why I have a case.

And I am certain many others would gladly trade their bulky, overpriced cases and bumpers for a sturdier device that inherently provides the protection we now have to purchase separately.

If you want a built-in case, you can get them. There is a whole collection of "ruggedized" smartphones from various manufacturers in China that are large, usually have a hefty battery, and have shielding built into the device.

Look at Doogee for one such manufacturer.

https://www.doogee.com/

Oukitel for another:

https://oukitel.com/

Ulefone for another:

https://www.ulefone.com/

Personally, I think that the built-in case isn't very interesting relative to a removable case, but the large battery might be, depending upon your needs.

EDIT: A number of manufacturers will even make official cases for their phones, if you can tolerate a removable case and just want something endorsed by the manufacturer.

Apple, for example:

https://www.apple.com/shop/iphone/accessories/cases-protection

Or Google:

https://store.google.com/product/pixel_8_phone_case?hl=en-US