Iran is the lesser evil. A pox on both houses, but tying ourselves to the bigger devil (Saudia) has hurt the US and the west as a whole
If the USA stopped supporting Saudia in any and all ways and let Iran attack it, the next day you would see the Sunni powers of Egypt, Turkey and Pakistan all jointly attack Iran
If the USA and UK enforced sanctions on Saudi, oil prices would rise, but this would barely affect the marginally surviving countries -- they aren't as dependent on oil as the west.
There are hundreds of ways to influence a culture. Cultures change over time even without wars. Probably the best way for Saudi Arabia to modernize is to continue failing. Their entire culture is on a collision with reality. The price of oil in real purchasing terms will continue to drop as more and more oil comes on line that was previously unreachable for one reason or another. They have already agreed to let women drive. They have sent home a huge number of imported laborers. They are adapting their population to work in real jobs, some for the first time. (Failing miserably there, but time will improve their performance.) They are developing a huge trade city where few of their religious laws apply. Most likely, the water for that city will flow from Israel. They are seeking foreign investment which means they must alter their culture or it won’t occur. The resent arrest (for “corruption) of dozens (or more) of Saudis who were held until they released some or most of their assets was more than likely a move to curb support of terrorism. (It resulted shortly after Trump sent high level emissaries to SA.)
Just attacking or allowing an attack on Saudi Arabia could cause chaos in the entire world. No war goes according to plan. It always gets messier and lasts longer than was envisioned. Recall, in almost every conflict the politicians promised the soldiers would “be home by Christmas.” Not only is the risk great, as many of those countries have acknowledged nuclear weapons or may, like Saudi Arabia, have purchased nuclear weapons, but the fallout, both literal and figurative, will be widespread and worldwide. The kingdom is moving in the right direction. If Trump were to initiate, in effect, a war, he can kiss his next term goodbye. It’s just not worth the risks.