The thing is US has not disengaged yet, and if the effort against Russia fails it would be one big blow to dollar. That’s what makes this move so audacious. Let’s see how US responds to MBS, but that guy surely has balls. Not sure if it’ll do him any good though.
The trend is very clear. They left Syria unilaterally even though they and the Saudis operated there together, they let the Russians and Turks in. Then they almost pull out of the military collaboration with them in Yemen (trump vetoed the senate). They destroyed Iraq and pulled out without leaving any mark, only for the Iranians to fill the void.
Everywhere they go they just end up leaving, not before destroying the country sovereignty, and giving a free pass to any country that want to swoop in. They are just not a dependable ally, unless you are Israel (and even here there are voices that we should limit our dependency)
The west in general is not a dependent ally in this parts. Lebanon was dependent of France and look at them now, from the crown of the middle east to a dump.
Also autocratic countries know they can "trust" other autocratic countries, there is a long term commitments and stability of interests that you just can't have with a democratic ally who is influenced by internal political considerations.