QLC store 4 bit or 16 charge states per cell while TLC only tracks 8, this means TLC simply reduced the workload placed on each cell because it track less states. As a result, although it allows more write operations, thus more durable, its capacity will be less than QLC. This means at the same cost you can get higher capacity out of QLC, or at the same capacity, QLC will cost less.In short, QLC is good for read intensive application while TLC is better for write heavy application. However most real world application are read heavy, i.e. you read much more time than you store, for e.g. playing music, viewing pictures, watching video, installing applications etc for the end user.
NAND on the market increasingly place less emphasis on the write performance, more and more SSD over the years are rated less than one drive-write per day (DWPD), a metric that tells you what percentage of an SSD’s capacity you can write to the drive each day over its warranty period.
While TLC won't be obsoleted, an SSD can be made from mixed TLC and QLC NAND. And YMTC also make TLC NAND, such as the 64 layer 3D nand that enter mass production last year. Just that by its nature, 128 layer TLC will store half of 128 layer QLC, whether to make 128 layer TLC depends on market demands, is 2-5 DWPD at 512 GB better than 1 DWPD at 1 TB? If you don't write 1-2.5 TB of data everyday why would you want 2-5 DWPD at 512 GB?
As for the fabrication, the 2D planar NAND has long reached its ceiling, which is why they moved to 3D layered NAND. You can't go for higher density on 2D because issues started to crop up, the physics I won't describe here. The challenge is instead of going down, how you can stack up, and 128 layers is state of the art on the market right now, ceiling of current technology. To go higher up, those are in research.
