Companies have done that since the dawn of time and will do so until the end of time. Did you have a point?
That performance inconsistency is inherent to all NAND flash, it is merely amplified the higher the layer count gets. The mitigation is NAND parallelism and having a large enough portion of your xLC NAND running in pseudo-SLC mode, neither of which this drive has.
NAND endurance remains a non-issue for anyone in a non-commercial scenario, regardless of the NAND type.
This has nothing to do with the controller.
Any drive based on non-SLC NAND loses speed when its pseudo-SLC cache is exhausted. Drives that lack DRAM for the mapping tables are consistently slower than drives that do include DRAM. Different controllers and different firmware has different performance characteristics. None of these concerns are specific to QLC.
There is only one reliable solution for cold storage, and that is the cloud. Physical devices, whether they are SSDs or HDDs or optical, are all susceptible to degradation.
As for SSD lifetime data retention, I suggest you read
https://www.anandtech.com/show/9248/the-truth-about-ssd-data-retention - it was published half a decade ago, and even then the estimated retention was 10 years without power.