Primarily talking about for NAS/Enterprise cold storage. Right now the normal pricing for consumer 1TB SSD's are around $250, and the pricing for consumer 3/4TB 3.5" disks is $90/$125. So SSD's are still around 8X more expensive per TB, but that's come down a LOT in the recent years. Extrapolating it out, it seems around 2020-2022, consumer SSD's should be the same $/TB as 3.5" spinning disks.
My biggest gripe with SSD's for mass storage purposes is actually the speed... it's unnecessarily too fast. I just need disk space and really don't give a crap about R/W speeds, as long as they're > 100/50 MB/s (which is around decent eMMC level) in parity-2 configurations. I'd imagine I'm not alone in this, as most NAS drives seem to be 5400 rpm or an adaptive 7200 rpm configuration.
Would it be cheaper for manufacturers to make larger, but slower SSD's? Or is that just irrelevant?
Realistically, I don't think they will "replace" them, at least not using current "flash" type technology in the near future. "Extrapolating it out" doesn't really work in a straight line any more than it does for modern CPU generational gains based on previous annual +30% boosts from the late 90's. Eg, Samsung have been the leader in 3D NAND, yet their prices of 850 / 950's aren't exactly plummeting compared to competitors previous 16nm MLC, nor are they rapidly stepping up the layers from 32/48 to 64/96/128/256, etc. Unless they do that, the process node "reset gains" from the 3D-ness are a one-shot deal, not something that can be factored in annually in a compounding manner.
Biggest problem for using SSD's as cold storage is the inherent flaw in flash technology itself (loses charge over time in an unpowered state 10-20x faster than a magnetic field on a HDD degrades, making them the least suitable storage option for mostly powered off cold storage of unchanging stale data). As cbn mentioned, QLC vs TLC improves cost at the expense of speed (as does TLC vs MLC) but the side effect also makes it far less durable for unpowered drives than MLC vs voltage drift over time, with almost no overhead in near-overlapping voltage states:-
SLC = 2 voltage states and 1x voltage threshold (100% vs 0v)
MLC = 4 voltage states and 3x voltage thresholds (100%, 66%, 33%, 0v)
TLC = 8 voltage states and 7x voltage thresholds (100%, 86%, 71%, 57%, 43%, 29%, 14%, 0v)
QLC = 16 voltage states and 15x voltage thresholds (100%, 93%, 87%, 80%, 73%, 67%, 60%, 53%, 47%, 40%, 33%, 27%, 20%, 13%, 7%, 0v)
16nm TLC already is at the point where barely 3x atoms per voltage state are holding the charge and where endurance has fallen from 100,000x to barely 1,000x P/E cycles. The problems we saw with Samsung 840's / BX200's falling to below 50MB read speeds as a result of struggling to read back what it wrote without extreme error correction were never actually fixed, merely a workaround that hid the issue by constantly rewriting data, "acceptable" for system drives booted daily, but that's no good at all for drives powered off for months at a time. 40nm 3D-NAND TLC has improved durability to match 16nm MLC, but as mentioned unless Samsung start upping the layers annually, it's a one-shot deal, and 40nm 3D-QLC is likely to be same or worse than what we saw with planar 16nm TLC Samsung 840's / BX200's...
Even today, the only SSD I'd even think about using for unpowered "cold storage" is the 850 PRO (40nm MLC), but the 13:1 cost vs 2TB HDD's makes that utterly pointless, especially since any serious backup strategy / NAS requires at least two drives. This is the number one point the
"HDD's are dead solely because predicted future 1:1 pricing trend" crowd miss - you can get away with 1x SSD for a system drive, but anyone who runs a NAS / manually "staggers" 2-3 external backup drives A-B / A-B-C, etc, obviously needs more than one drive.
Unless there's some radical breakthrough that replaces flash technology with something similar but doesn't lose charge over time, I don't think even 10-16nm QLC will reach price parity before we're well into the realms of "junk durability" for unpowered offline storage.