There a lot of applications for which a MCU is sufficient and it will be much cheaper and it will have a much lower power consumption.

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.

Correct.

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).