Saudi Arabia benefits just as much, if not more, from a diminished Iran.
Iran has funded terrorism and religious extremism in the region leading to massive death tolls in many of them. Not to mention Iranians themselves.
Oil funds predominantly along with theft and ransoms according to the EU near ISIS's peak [1]. This was while the US was still directly fighting ISIS in Afghanistan and Iraq.
1: https://www.europarl.europa.eu/RegData/etudes/IDAN/2017/6038...
You need to be a bit more specific with exactly what you mean by “religious” extremism, you seem to be conflating things that are in many ways opposite