They were never doing anything directly to remove him. They were simply arming and supporting militant rebels who wanted Assad out. Lots of these militant rebels have since joined ISIS and have overtaken a large part of the region which includes oil fields.
That is a well-documented fact, not a conspiracy theory.
This oil is being purchased, and I can assure you it isn't Russia or Assad buying it. That is exactly what a Russian jet was doing in that region- scouting for black market oil buyers who are facilitated by the Turks. The Saudis definitely don't need the oil so.
USA/NATO cannot directly attack Assad- that would be "an act of war". They can no longer directly arm the rebels who are now a part of ISIS either. Instead, they turn a blind eye to Saudis and Qatari billionaires funding and arming ISIS. And my guess would be that in exchange they get the oil from the oil fields while overthrowing Assad. Win win situation for them.
Only because Russia and China vetoed the UN resolution to send NATO forces into Syria to remove Assad. So instead they indirectly support foreign mercenaries to achieve their objective.
They also tried the false flag chemical weapons red line in the sand bullshit so they could launch direct attacks, but Russia outplayed the yanks diplomatically by getting Assad to destroy his chemical weapons stockpile.
They have tried to do it directly, but haven't had much success. Now they just support proxies to get Assad removed from power.