My goal isn’t to replicate drive functionality - the relatively affordable and extremely popular SD2IEC already does that. There are other solutions which offer better 1541 floppy emulation and speed also.
One of them is indeed pi1541, a Raspberry Pi based cycle exact 1541 emulator. It’ll work with fastload solutions, but obviously it’s still limited to the slow speed of the IEC bus.
In contrast, something in a cartridge or maybe user port could be one or two orders of magnitude faster. This wouldn’t be “compatible” with normal floppy drive loading/saving of data. It would be some sort of special addition that new software could take advantage of. Or old software could be modified to take advantage of it.
The point is - if it’s cheap enough than many users would buy it even if it doesn’t initially have much software support.