I think the utility is more with experimenting with distributed computing technologies in a relatively affordable way, with hardware that is familiar and simple to set up.

I made a small SAN, just for learning. Four pi4's serving two USB-SATA disks each via iSCSI to my NAS, which mounts them as a ZFS pool. The pi4's are connected to a switch with a 10G link to the NAS.

Again, mainly just for fun and experience.

They are much faster because they draw more energy.

The Raspberry 4 at 1 Gflops/watt is a better option if you want to power the thing with lead acid batteries as backup power for more than 24 hours.

Also Raspberry 4 can be cooled passively with a smaller heatsink so the total size and weight is less.

PI's are not really power efficient - their CPU is made on super-obsolete 40nm node. They have no sleep mode, or any low-power mode for that matter. Imagine how long a phone would last on that battery - its the same class of CPU.

An Intel compute stick or beelink mini pcs are also passively cooled and weigh no more.

Nope 28nm for the Raspberry 4. And 5nm is not going to save you that much energy compared to 28nm, it will however make the circuit much more fragile.

The Raspberry 4 is peak power efficient: http://web.eece.maine.edu/~vweaver/group/green_machines.html

Mobile phones don't have a keyboard, you can only consume with them. I produce things.

Those sticks don't have a company with it's own GPU and linux (Raspbian now RaspberryOS) behind them and they cost more per watt for performance I don't need.

However I have 2x NUC with Streacoms passive case: https://streacom.com/products/nc2-fanless-chassis/

28nm vs 5nm, is what, 3 generations with a 20-30% power saving at each step. ~50% power reduction is something.

Nope, if you compare theoretical Gflops/watt it's probably 30%, but in practice you'll only get maybe 15%. Memory speed is the bottleneck and you cannot get around that. DDR3 had lower latency than DDR6 and only bandwidth is increasing, and you have to ask bandwidth for what?

Sorry, I don't follow. Why would anything be getting faster if we are talking about hypothetically utilising (just) the power-saving gains from newer nodes?

Basically Gflops/watt is not going north of 1 for 32-bit CPUs, so humanity is completely out of growth. This is the end. Game Over!