"Good loudspeakers have a flat listening window..."

The target LW flatness is a function of the radiation pattern of the speaker. You don't want an ALFR to have a flat LW because it will sound too bright and also the depth of the soundstage will collapse.

"Big rooms requires big speakers to avoid audible distortion."

Those who listen closer to the near field for a more intimate presentation or don't like bass, may not need big speakers.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated