Can an SD card (or flash drive in general) be made Read Only? The old floppies had a mechanical "switch" that could be flipped to make it impossible to write to a floppy. CDs are of course by nature Read Only. It only makes sense to distribute installation software on Read Only media.
If SD cards can be made Read Only, I kinda think that Apple may be going that way. It's not just the cost of the card, but also the cost of packaging and shipping. Plus, Apple may be thinking environmental issues too. I don't know what the green credentials are SD vs CD, but there might be some motivation there.
If we continue to see SD slots added to Macs, then I expect there is a good reason.