You want the volume of air flowing through the straw to remain constant so you cover part of the end of the straw with your finger as you start to blow harder. The harder you blow, the smaller you make the hole in the end of the straw. As the pressure going into the straw increases and approaches infinity, the size of the hole has to approach zero to keep the volume constant. The pressure in the straw near the hole also approaches infinity. The pressure drop across the length of the straw approaches zero. You can make the straw larger or the inside surface more rough, and the pressure drop through the straw will still decrease as you increase the pressure in the straw.
