The rise of 3D printing would also make the use of old shells only for nostalgic reasons. One can manufacture a shell in any novelty design, as long as the business end of the cartridge lines up with the slot, and do so very inexpensively. If you use the right materials, (Like ABS), you can even make it smooth and shiny. (see also, vapor smoothing.)
I suppose that with certain sports titles, you also get the NVRAM chip and battery, as long as the NVRAM is suitably sized... That means programmed eeprom replacements on the game's PCB would be a thing.
Outside of selling DeJap'ed RGP titles that never got sold in some markets as bootlegs, I would think that the best economy is still one of those super piracy/developer carts though. The kind that have a full FPGA in them attached to the cartridge bus, and can do ANY title, even those with special CPUs.
But that's a dangerous flirtation with ban-worthy territory.
Hell, with a 3D printer, you could make a custom shell with signal cutoff for one of those super cartridges so it can live on the EXT port on the bottom, with a load resistor based disconnect, so that when a real cartridge is plugged into the top, it uses that instead. I would think that would be much more interesting than re-purposing old Madden carts.
(The EXT port is basically the same bus interconnect as the cartridge slot. It was engineered in as the means to attach the vaporware CD drive to the system, and was never used. Theoretically, a few load resistors and some straight line traces to another card socket at a 90 degree bend would let you install a recessed cartridge that would get loaded if the cartridge slot on top did not have a cart inserted-- OR, even be a place to put a live cartridge dumper or cheating device. The design of the SNES exposed the full memory and IO interface, because it was intended to upgrade the SNES with the very cartridge you plugged in, if you needed custom processors and the like. That's how SuperFX and pals work. Theoretically, you could put ANY cart on such an adapter, but I think one of the super pirate cards would be the most beneficial.)