Well they could make 8tb drives easily in 2.5inch casing or on a pci-e slot drive.
The limitations are down to their own decisions.
I own currently 8 sata SSD's they would only be replaced with larger capacity models that match same durability/performance spec aka not QLC.
I own also 3 NVME SSDs, I think its a tough sell to me to replace an existing one with one that benches faster but has no real world benefit. The way to make me replace those again is higher capacity replacement without a nerf in existing metrics. This is especially the case for M.2, as M.2 is so expensive to place on a motherboard it has high real estate requirements, so limited M.2 slots means its basically high capacity only now for new M.2 purchases, my prime reason for the purchase of my DC-P4600 was the 2TB size to its cost.
It feels they pushing performance as the enticer to get enthusiasts to keep buying new models instead of capacity. I feel this is all linked to M.2 limitations. U.2 would have been so much better.
Image below to show limited board real estate for U.2 connectivity.
With U.2 drives been connected via cable it means the limitation for space is moved from the board to the case which is how it should be.
