If we put the economic part aside, what kind of storage density could we reach with these in a 4U case? I mean something like a custom build Backblaze Storage Pod [1] but for microSD cards.
4U-Rack volume: 450.85x(4*44.45)x800 = 64.128904e-3 m³
At 100% usable volume ratio that would be 555228 Micro-SD-Cards or 222 Petabyte and cost $55,000,000,000 (assembly and shipping not included).
The storage density of the cards is 2.77e19 bit/m³.
Assuming a saturated gigabit (100 MB/s) connection, uploading your backup would take 70.4 years.
555228 x $250 = $139M, not $55B.
You could use smaller cards, but then you have the nightmare of trying to find a stable supplier that doesn't have a ton of fakes in their supply chain.
Even then, your competition is 10TB HDDs, and you need 25 cards + card readers + cooling to match one drive. Performance is better, but probably not enough to justify the expense.
PS: Your lack of faith disturbs me.
A small SSD for the main OS - then a ton of space RAIDed on these.