I probably should keep out of this, given who is at fault for this topic coming up, but you need to consider that cassette hardware is growing harder to come by as well - while cassettes and cassette players are still manufactured, they are made in tiny quantities today compared to either media such as SD cards, or to the volumes in which they were sold in their heyday of the late 1970s and early to mid-1980s.
A new cassette tape would probably cost only slightly less that a 4GB USB flash drive (about $6 US where I am living) - keeping in mind that (in the US at least) a number of companies give flash drives of sizes up to 4GB away for free when passing around samples of whatever they happen to be promoting.
For example, compare this
Maxell normal bias 90-minute cassette 3 pack, currently priced $7 US on Amazon (unit price $2.33), to this
Verbatim 4GB USB flash drive 3 pack for a little over $16 (unit price $5.40 US). While the USB drive is more than twice the price, that's mostly because of the USB circuitry - the cost of the NAND Flash data storage components is only a tiny fraction of the price, which is why the prices for 256MB, 1GB, and 4GB drives are almost identical. For that money, you get over 6000 times as much data storage, at vastly higher transfer rates.
Now, according to Wicked-Pedo, the
Commodore 1530 Datasette ran at the same 300 baud as the
original version of the Kansas City standard; AFAICT, the relatively few cassette formats (e.g., Acorn CUTS) which worked faster with
standard cassettes still maxed out at only 1200 baud. In a USB drive, 1200 bits per second would be considered within normal performance variance (i.e., negligible).
Regarding capacity, a 90-minute tape could, with the most efficient encodings used, record 660KiB of non-compressed data, though if you absolutely intend to update this idea, I suppose it wouldn't be too hard to add Run-Length Encoding or even adaptive Huffman compression to tweak that a bit (though the resulting capacity would be data-dependent).
TL;DR: if you are expecting (as
Lee Felsenstein did when he was designing the
SOL-20 and the Pennywhistle modem in the mid-1970s - he deliberately tried not assume what replacement parts would be available later) to have to pull hardware out of landfills to avoid the Establishment's lockdown of new systems, you're likely to find that there are a lot more discarded USB flash drives than cassettes and cassette players. Just Sayin'.