That's the thing. Steam caught on in part because compared to physical retail "only" 30% was a massive improvement for the average game company. I've heard 60-70+% going to buying/printing floppies/CDs, packaging, distribution, etc. at the high end. I'm sure the big companies got bulk discounts or multi-year deals for supplies.
Yes that meant that since those aspects were a significant amount of the cost anyway you could do stuff like the StarCraft Battlechest that was crammed with extras. But it also did a lot of gatekeeping in it's own right.
Now you could probably do some kind of limited run thing that would be a lot more viable, but it's definitely a luxury step up.
I would be surprised if a cartridge being discontinued would kill an open source project like this. They could switch to a different cartridge.
I'd assume they're doing the one for now because it's cheap-ish and has widespread availability. There's little reason not to expand to other cartridges once the rest of the printer is more stable.