Funny enough, SANDISK already did QLC based SD cards way back in 2009. So it's not like the flash tech is not matured (it's literally just distinguishing 16 voltage states instead of 8 or 4) but rather ssd controller support is not there yet.
actually, neither of that is correct. The issue was defective chip substrate, where bumps inside simply disconnected. Heating the chip to 130c or so "fixed" the issue temporary. People thought it was solder, because solder melts too.
Thats not entirely true. Yes, you can extend NAND endurance by employing better error correction and compensation for degradation, but ultimately it's all down to quality of the actual silicon, how much cycles is going to hold up.