Russia simply wants to have a warm water port and wants a stable regime in Syria, as does China. Neither of them is going to tolerate a Global Caliphate, a resurgent Ottoman Empire, nor regime change in Syria without being consulted and between them they are strong enough to force the West to rethink and deal with them even taking into account America’s space command.
So unless we want World War III to happen earlier than planned, a grubby compromise is necessary.
Saudi Arabia, Qatar, Jordan and Kuwait allegedly want a global caliphate, the precursor to which will be the overthrow of the Assad regime.
Turkey wants to recreate the Ottoman Empire and allegedly take for itself that part of China populated by Uighurs and that part of Syria and Ukraine populated by Turkmen.
I suggest that ISIS cannot be eliminated without “boots on the ground” and very heavy air strikes which will kill both ISIS fighters and commanders and the civilians they hide amongst. The West is not prepared to undertake this on the scale necessary and therefore, has no endgame regarding ISIS although it pretends that it does.
Britain probably should bomb ISIS in Syria to keep faith with its allies, but we should not pretend that our objectives do not conflict or that we can destroy ISIS on the basis of what has been proposed so far.