Increasing power required with altitude.. what's a good plain english explanation?

Who cares about IAS? The question was does it take more power to go
faster, right? Any non pilot will think faster means true airspeed, not
indicated.

True, but the conversation got to how high a plane can fly. I said that
going higher did two things: limited the amount of power that an engine can
put out because of density, and that even if you had an engine that didn't
lose power, the power required goes up regardless.

Also, not that it would matter to a non-pilot, but IAS obviously matters for
keeping best range speed for instance.