Long ago this blog said there could be no peace in Syria without engaging Russia and Iran. At last this is now happening. Unfortunately there remains a gap between the two sides (who agree in principle on a good deal and enough to build a political settlement) over the future of Assad. The West supports the tribal view held by many Arab states that his departure is a condition of progress. Russia and Iran have a more pragmatic approach driven by the priority of not allowing the State of Syria to collapse. They see too many failed states in chaos for that to be allowed. Assad must therefore remain until political negotiation brings a settlement. Then he can go.
It should be possible to bridge this gap somehow. It is not a question of Assad staying on; rather for how long he remains the titular head of the country. In practical terms much of his authority is now with Russia and Iran. The West and its Arab allies may choose to disagree with both, but Assad is in no position to disagree with either.