If it looked like some force would take over Saudi Arabia, Europeans could step in to defend the Saudis or Saudi Arabia could buy what it needs to protect itself. Europe can get their contracts for oil elsewhere, perhaps from us, and the oil from Saudi Arabia is sold to Russia or China or whatever.
It’s all the same basic commodity and even if the Saudis are taken out, we have plenty here now that we never knew before.
I don’t see the problem, still.
If they are strong enough to dislodge the House of Saud, they probably are a larger or stronger entity than an Islamic group like ISIS.
Anything larger than that that triggers massive European intervention might be the start of World War III.