Kurdish forces are the only reliable ground forces the US have in Syria to defeat ISIS, so the US will continue helping them till another ground force is willing to take on ISIS. Lets say if Turkey launches a ground invasion in Syria to take out ISIS, then ISIS will simply retreat to Iraq. So if there is a Turkish ground invasion in both Iraq and Syria to defeat ISIS, together with a plan for a democratic union of Iraq and Syria under a GCC, Arab League and Turkish peace keeping force, then the neighborhood can be stabilized and the Kurdish threat can be neutralized for the long term. The longer Turkey waits and tries to avoid the problem instead of tackling it head on, the more painful and unreachable these options will become in the future.