In theory flash can be use for archival but it would have to be refreshed periodically.

The cells are still intrinsically more fragile, and increasing the bits-per-cell increases capacity only multiplicatively while retention and endurance decrease exponentially; theoretically, to a very rough approximation, 4 bits per cell will have 4 times the capacity but only 1/16th the endurance and retention of 1 bit per cell. In practice, it's somewhat worse.

Instead of a 1TB card using 4bpc flash that may offer 1K cycles per cell (1PB total writes), I'd rather have a 256GB SLC (1bpc, "old-school" flash) card with 16K endurance (4PB total writes). Both take up the same area (thus theoretically cost) and the SLC is in fact simpler in many ways to use because it doesn't really need such elaborate "management", but unfortunately in practice SLC is priced astronomically higher than the technology would suggest.

I'm not much for conspiracy theories, but it sometimes makes me really think if the industry is deliberately misleading us into the worse choice in terms of reliability, just so they can sell more. This is one of my big gripes about the flash memory industry --- there is a lot of marketing, a lot of fluff. But if you look at the facts it's clear that no one considering the tradeoffs logically would've ever thought MLC to be in any way better. It can certainly be two, three, or four times the density of SLC, but it doesn't even last a half, third, or fourth as long.