I haven't been following gaming recently so apologies if I'm out of touch but how are Nintendo rom sizes calculated purely by saying they will be the same size as the download up to the nearest cartridge size or is there some sort of site that lists actual rom sizes? I had a quick google but couldn't find such a site.
For some of the older formats the download size was larger than the rom size, often more heavily encrypted but optimised for faster loading so took up more space.
Some of the Switch backups appear to be from the download file not the rom too which confuses the issue and I can't seem to find any obvious site that has a list of rom sizes. It looks like gbatemp might but isn't currently working or maybe it has been taken down.
Going back to the main subject matter surely if retail makes up such a tiny amount of sales they could if they wanted to even take a loss on each copy sold if the number of copies is going to be very low. Maybe Nintendo pushed them to sell some copies at retail but its just a tiny amount and the vast majority will be downloads. The reality is there are huge profits with downloads and very little with cartridges, by restricting cartridge sales you still look like you are supplying that market but you are forcing many to go digital. $30 doesn't seem a lot, these are customised cartridges and with everyone taking their cut it's slim pickens surely with a 16GB cartridge.
