Realistically no modern consumer OS is ever at idle for long.

It's constantly monitoring WiFi signals, battery level, checking for background processes to run, and a hundred other things.

Whether CPU usage is being reported as 0% or 1% averaged over the course of a second doesn't have anything to do with poor design. It's just being rounded from values like 0.3% or 0.8% anyways.

Monitoring Wi-Fi signals is, afaik, something that happens on the Wi-Fi chip itself, not the CPU.

While you’re correct that nothing stays truly idle, the modern design is that the main CPU really does stay largely idle because of the power costs involved and instead dedicated microprocessors absorb the load when possible.

Sure, but most modern OSes would inform the user when the WiFi connection dies - so there's something happening on the CPU too.

On a state change, sure. But you're not frequently gaining & losing WiFi access.

The only state where a modern computer is "idle" is when it's turned off.

You can’t really make this claim without measuring the actual impact, especially on a system with frequency scaling and 8+ heterogenous cores.