To be frank, I don't get its appeal at all. Most hobbyist uCs were already unbrickable, and using mass storage mode for flashing is rather cumbersome and clunky to automate as soon as you're past hello worlds.
However, the appeal of mounting as a mass storage device is not for iterative development (as you mentioned). Invariably something breaks, and the easiest way to get back on track is to reflash their default blank firmware using the mass storage interface.
UMS seems like a nice idea when you first hear about it, but it doesn't really offer much once you look at it closer. The only proper argument for it is "no special software needed", but it a world where picotool is `apt install picotool` away that's not very advantageous anyway and only causes automount annoyances.
UMS shines where you have a device with a filesystem in its flash that you can access to actually manage the files stored there. Super useful for stuff like MicroPython. In contrast, pseudo-mass storage like on RP2040 doesn't seem very useful at all. It makes it appear more approachable, but only superficially.
(It just takes a long time for the OS to recognize a USB device. And you have to press the reset button yourself in order to enter the bootloader. With something like J-Link, your build script can handle pressing the reset button and sending the code, saving you quite a bit of time between iterations.)
I make open source espresso machine hardware (github.com/variegated-coffee), and it's nice to be able to give users a wired way to update firmwares that doesn't require extra hardware.
I just think that despite all testing and care bugs are still possible, and the ROM bootloader is a backup that's always there. Plop a tiny switch on the PCB, and even if I screw up an OTA update customers will still be able to flash with no special tools (if the device has a USB port, that is).
I also use it as a recovery state for panics, makes the device impossible to brick by a panic loop.
That's orthogonal to mass storage mode. ROM bootloaders were standard in this class of microcontrollers for years, but they usually don't use UMS. One could argue that UMS is perhaps better than some custom incompatible solutions, but then that's what DFU is there for - a standard way to flash things over USB.
Small to medium production runs are always a bit of a pain, because you have to do a lot of coordination with the factory when it comes to tooling. You have to ship a custom programmer, get them to install the drivers on whatever OS they are using, and then find a way to write custom code to interact with the programmer and deal with all the possible error conditions.
The RP2040? A simple script which detects the presence of a RPI-labelled flash drive, copies a file, and repeats. Written in half an hour. Drivers? Not an issue. Hardware? Everyone has a USB cable lying around already. Error conditions? It either succeeds, or it doesn't - the OS handles the rest.