Trying to topple Assad now makes no sense (if it ever did)

There are now two aspects to the US-led approach in Syria: first to topple Syrian President Bashar al-Assad from power and, second, to use Western air power, coupled with local forces on the ground, to inflict a decisive defeat on ISIL.
ISIL fighters will not stand and fight with a conventional military. What they will do is blend into the towns, the countryside and cities and return to fight another day, so there is no military solution to defeating ISIL.
The problem with this approach is, now that the Russians, Iranians and Hezbollah are committed to keeping Assad in power, it's impossible for the US, and the West more generally, are going to topple Assad.
Instead a situation arises where the Russians and the US fight a proxy war over Assad which must detract from the American-led efforts against ISIL. To say nothing of the efforts of US satrap states, France and Britain.
Also, ISIL is an ideology as much as a military force.
There is no question that if the US and the European powers put a large number of ground forces in Syria and Iraq they could defeat ISIL in a traditional war, but the problem is that ISIL fighters would just evaporate.
like the Taliban did. Two results -

• as long as the West occupied Syria and Iraq more terrorists emerge
• once the US and its allies left, ISIL fighters would come back as Taliban fighters did in Afghanistan.

Assad is there to stay as long as Russia backs him, which is a situation which will continue.