I've been waiting with bated breath for a SATA 3.5" SSD with high capacity.

I might be waiting forever, because clearly there's nothing coming. Though I'm not sure if it's because it's technically difficult (high power consumption to keep the flash lit?) or something else.

I'm aware that it leaves performance on the table for the chips, and probably that means that unit economics means that for the yeild: OEMs would rather make high performance drives which sell for more.

But a 4-bay NAS with 3.5" SSD's would be silent and theoretically sip power, and there so much space for chips, you could space them nicely and get 10+TiB in a drive...

I don't need to touch every cell, I just want something silent and stateless and less power intensive for my time-capsule backups and linux ISOs.

Alas.

There's just no benefit to using SATA for it. Even a PCIe Gen3 x1 link provides a significantly higher performance - and those aren't exactly rare these days. Why invest a huge amount of time and money into building a controller chip which is significantly worse than its competitors in every way? Even if you're very interested in backwards compatibility it would make more sense to go for PCIe-based U.2 instead. And 3.5" is just a waste: look at 2.5" SATA SSD teardowns, they are mostly empty space.

If you really want a classic hotswap bay form factor, something like the Aoostar WRT Max allows installing a bunch of M.2 SSDs in a 3.5" bay. The QNAP TBS-h574 gives you five swap bays for M.2 SSDs - albeit in a cute custom form factor. Just want a whole bunch of storage? The Asustor Flashstor Gen 2 has up to 12(!) screw-down M.2 slots. At 8TB per slot that's 96TB of storage - significantly more than the 40TB 4-bay NAS you are proposing.

Or wait for someone to build a NAS which accepts EDSFF SSDs. But don't count on it, because there's no market for prosumer-level EDSFF drives (nobody has a bay for it, and M.2 is a far more attractive option for most people), so there's no market for EDSFF-compatible NASes either. Unless you plan on shipping them with M.2-to-EDSFF adapters - but at that point why not save a whole lot of space by going directly for M.2 instead?

It's a bit like asking for a Mini-ITX motherboard with an SP5 socket so you can plug in the latest Epyc CPU, and wanting it to have DDR3 memory slots: even if it's technically possible at all, what market is it supposed to serve?