The fact that these are available with 4 and 8 GB of RAM now and with Pi-compatible GPIO could mean some excellent home project opportunities.
I've been watching for an SBC you could use to make something like a TRS-80 Model 100. They ran off 4-AA batteries and would get you 20 hours of continuous use. That translated to at least a week of real world use (you wouldn't want to be typing on one for 4 hours a day...)
That was in 1983. Today, it seems like you should be able to make something similar that will run for weeks or months on AA batteries.
So in my very-much-not-an-expert opinion, I don't think it would be feasible to run a modern computer off of AA batteries, let alone for weeks.
The idle power consumption of such a SBC like Radxa X4 must be significantly below 5 W, probably around 2 to 3 W.
That is still too high for standard AA batteries, but rechargeable batteries should work.
If a long battery life would be needed, it is likely that one can shut down the N100 CPU and wake it up either periodically or on USB activity caused by the RP2040 MCU. That should reduce the average power consumption to fractions of a watt.
There are all kinds of Linux Capable chips available between 0.3W to 5W, and with LPDDR1 or other older standards, you can get very good and cheap prices while still only sipping power.
Much higher than a raspberry pi, but still low in comparison to any desktop server
Most well designed N100 systems that have only a moderate number of peripherals have an idle power consumption under 5 W.
A modern MCU will absolutely run circles around the processing and memory capabilities of the Model 100, and can probably even do that for 20 hours using four AA batteries.
However not even the fastest existing MCUs, i.e. those with Cortex-M85 cores, can reach processing or memory speeds comparable with CPUs like the Intel Alder Lake N series or the Arm CPUs using Cortex-A78 or Cortex-A76 cores.
The fastest MCUs may have a clock frequency of up to 1 GHz, but most of them have clock frequencies many times lower (this is because the MCU cores use shorter pipelines), while the SBCs use clock frequencies between 2 and 4 GHz.
The Cortex-M7 or Cortex-M85 MCU cores have an IPC (instructions per clock cycle) that is 2 to 3 times higher than the IPC of most other MCU cores, but even their IPC is 2 to 3 times lower than of Alder Lake N, Cortex-A78 or Cortex-A76. Due to the much higher clock frequency and IPC, the SBCs are much faster than any MCU.
Due to the MCU clock frequencies being under 1 GHz, not even their cache memories can have throughputs as high as the DDR memories of the SBCs, which use 4.267 to 4.8 giga transfers per second.
So any modern SBC will run circles "around the processing and memory capabilities" of any MCU. However, you are right that in many cases the speed of a SBC is not needed and a MCU is good enough.
A SBC is typically needed when you want a USB 3 interface or an SSD, because extremely few MCUs have any peripheral interface faster than 1 Gb/s Ethernet.
But in context: It simply can't take very much grunt to provide a modern take on a ridiculously-limited portable computer from 1983.
The TRS-80 Model 100 that is the context here had an 80C85 processor running at a scalding 2.4MHz, and featured as much as 24 kilobytes of SRAM.
And that's a pretty excellent spec for a portable computer in 1983, but it's a complete snoozefest compared to a RP2040 dev kit like the Pi Pico (at a cost of around three US dollars).
This board is a full intel computer plus a rp2040 driving the pi-style GPIO pins. It should basically be able to do whatever a pi can do.