QLC would be the natural progression from TLC, and would lower the GB/$, fact.
However, just as the write endurance has been getting more focus lately when moving from SLC to MLC to TLC, data retention should be a parameter getting more focus moving onwards.
I can not recall reading about data retention in consumer HDD/SSD reviews ever! (My memory spans about 5 years or so, so leave a comment if you have).
Apparently these electrons and atoms that store our 1 and 0 are in the habit of taking a walk'about, and we don't want that!
I wouldn't mind getting a QLC flash based storage unit for archive purpose, if I knew that, the device, left untouched, still retained the data of all the cells after 1 year, or perhaps 5 years.
Ps. I do know that the first CD I bought still played after some 15 years (Depech mode: Enjoy the silence!). How it fares today I don't know; I'ts been converted into flac, sitting on a HDD... Ds.