The question becomes “why are there no (or a lack of, I’ve never actively sought SLC SSDs) SLC SSDs?” I think the answer is very simple. MLC/TLC/QLC/PLC brought prices down for high capacity SSDs. Who needs say a 250GB cutting edge performance SSD? What does M.2 peak out at these days, something like 15GB/sec? Wow, I filled a 250GB SSD in 16 seconds! If you’re going to have that kind of throughput in a product, it’s got to end up getting stored somewhere, and that item has got to be affordable. Most high-end mainstream SSDs I’m aware of are TLC tech, so let’s do a simple calculation. If a 1TB TLC M.2 drive costs £100 then logically (ignoring physical and supply constraints) a SLC equivalent drive is going to cost three times more. When a single ‘current’ computer game weighs in at over 250GB in terms of storage needs, if one wants more than a few games installed then one needs at least 1TB SSD storage. How does that affect the cost of ownership for the average PC gamer? I (for personal reasons) have never spent £300 on a storage drive. £150? Sure (my first SSD: Samsung 840 PRO 256GB).
I personally haven’t noticed any real-world difference from changing from a decent SATA SSD to a decent NVMe drive. Why do I need to be able to do multi-gigs per second transfer speeds until the cows come home? At my business’s end of the market, I go for decent quality SSDs for two reasons: 1) I’ve seen what happens in terms of real-world performance with DRAM-less SSDs, 2) I’m hoping that buying from higher in the range than the trash results in better quality and reliability.
I’m going to ignore your capacitor question as I know next to nothing about SSD manufacturing techniques.