Why not let Russia get bogged down in Syria and spend all kinds of money on fighting "terrorists" and then more money to rebuild if they actually win?
The United States has no clear strategy in Syria. We are spending billions and don't even know what we really want for an outcome, except perhaps for the removal of Assad. This is probably not going to happen as we are not sending troops and Russia supports Assad. Sure, we want to get rid of ISIS, but we're not doing that very well.
Why not make a deal with Russia that allows those who want to leave Syria leave; agree to secure borders of surrounding states; and let Russia get deeper in the whole with ISIS? Syria is of no particular strategic value to the U.S. Let Russia create another Afghanistan/Vietnam that will keep them occupied for many years to come and save us lots of money. Assad will eventually self-implode and Russia will have to clean up the mess.