In an excellent leader that Economist argues that the longer Russia and Iran keep Assad in power, the more they will share his guilt for war crimes.
Source: A poisonous client: Why Russia and Iran should ditch Bashar al-Assad | The Economist
It looks like the article was published before President Trump authorized the destruction of the Syrian military airbase that was responsible for the latest gas attack on the Syrian people. Russia was kept in the loop with advance warning.
It remains to be seen whether Trump will signal a wider change in policy in the Middle East.
One can only speculate that the US could offer Russia richer pickings than Syria.