Sure we can. Instead of 2, 3 or 4 NAND chips on a 2280 form factor, we use 4, 6, 8 or a few more to get higher capacity SSD's in a 2.5" 7mm form factor.
Sure, as incedental storage in external/usb drives, not as boo/OS drives.
Hint, it's not any different. And again, QLC durability in P/E cycles is pathetically low.
That's an opinion. SATA4 would be a solid way forward in terms of compatibility and continued expandability.
Cost is not as important as durability, performance and capacity. I would rather spend $200 on a 2TB MLC drive than $100 2TB QLC drive. All day, any day.