If they wanted a regime change, it would have happened. They control major oil fields and stealing millions per day from it. Israel claimed Gaolan heights and massive reserves there. They've prevented pipelines through Syria that bypass Israel's monopoly that currently feeds all of Europe and into Asia. The continued conflict prevents stabilization of region and hides their crimes. It forces millions of refugees into Europe and America. It clears out region to allow more and more annexation. Control of a depopulated area is much easier than subjugation. Force them out first and take piece by piece. Then there's financial aid stolen from west and constant spending to drive the debt based economy and hide wealth transfers. ISIS literally publically apologize to Israel for accidently attacking their soldiers. Mass organ harvesting of injured Syrians and Palestinians drives Israel organ trade. Opium field control.

This and more are the reasons the regime change hasn't happened. If they wanted Assad dead, it would have happened very easily. He is a known commodity and can put blame and evils on. He can be blamed for gassing his own citizens when ISIS is retreating and Americans are withdrawing and he's basically won and magically they now have a new reason to stay.