It doesn't need to be able to reach 1PiB. There are plenty of 120-128GB SSDs with MLC NAND that won't hit anywhere near 1PiB. As long as the Samsung 840 120GB can do 120TB (which means it meets Samsung's promise of 1,000 P/E cycles minimum), it's good. I used 50GB host writes setting up a new PC with Windows 7. At that level of usage per day, the 120TB minimum NAND writes will be used up, in what, 6.5 years? Most folks only write an average of around 5GB, at most 10GB to their SSDs per day. At 10GB/day, you'll use up the 120TB NAND writes in 30+ years.
There are plenty of TLC NAND manufacturers. The problem appears to be none have quite the same expertise as Samsung when it comes to controllers and firmware. Samsung really did something pretty amazing with Samsung 840 TLC. The only unfortunate thing about it is pricing.
The Samsung 840 tends to be more expensive compared to previous gen models with MLC NAND, particularly the Samsung 830 during all those clearance sales. The move to TLC NAND is designed to drive down prices. Unfortunately, prices were already down when the Samsung 840 was released. You see sales on previous gen 120GB MLC NAND SSD for $50-80 quite often, but the cheapest I've seen for the Samsung 840 120GB is $80 ($70 after $40MIR). Normally, it goes for around $100 or so. I've seen the Intel 330 and SanDisk Extreme 240GB for $140 on a couple of occasions but the lowest I've seen for the Samsung 840 250GB is $150. However, there have been some pretty good deals on the Samsung 840 500GB (B&H $300, Adorama $310).
Also, the fixed costs (packaging, chassis, PCB, controller, RAM, etc) for all the capacities should be the same so shouldn't the higher capacity models be cheaper per GB compared to the lower capacity ones? Why do 480/500/512GB capacity models often cost significantly more per GB compared to 240/250/256GB versions?