For a very low power project I had, I basically found a lib that did deep sleep correctly for my microcontroller (an ATTiny), then you sleep for a significative period of time each time (for example, 500ms), and check if the event happened when sleep ends. That results in a consumption under 10uA.

It is obviously very dependant of what you want to do, some applications can't afford to check only every X ms.

And be careful too, you need to put everything possible behind your microcontroller, to be able to disable things during sleep (otherwise, you are just powering the rest of your circuit for nothing).

In this style of coding you end up with

  main()
  {
  configure_interrupts();
  while(1)
  {
     do_irq_bottom_halves();
     sleep(); // "wfi" in ARM
  }
  }

The basic idea is that it is possible to put the chip into lower-power mode... but you need a way to wake it up after that. That could be an interrupt, watchdog timer... etc

Another thing to note is that for most applications the mcu's power consumption will be negligible with respect to the peripherals', so the average person shouldn't care much.

I have one application where the CPU literally does nothing other than process switch actions (it turns a double click into a single click), so it goes into deep sleep and pressing a pushbutton is the only thing that wakes it up.