Iirc, and I could very well be wrong, but a cell/page/word is marked by an SSD’s firm ware/controller as bad when it no longer reads a reliable, or accurate voltage from it.
The idea being the P/E cycles are slightly destructive, and the cells break down.
With an SLC, the drive only needs to have X charge stored, and if a cell does not hold it, it gets marked as bad.
Deeper levels require differing voltages, so perhaps a QLC drive would hold up well, even though the quality might be a bit less, and the chips are much more dense, and being a smaller lith, there is less actual material, But as the cell was going to store a wide range of voltages, the sensor might be more accurate, and the cell might not have to store As high a charge/voltage to register as a 1, or 0 and so might benefit from that as well?
So I posit, and am happy to be told I’m wrong, but IF the read sensor is more accurate, And the cell requires less charge to set, the the chip, and hence drive, would probably be cheaper than an SLC with similar capacity, and last a similar time? Even though new QLC chips might be a lot less durable.