The Syrian Kurds would like to negotiate a trade route to the Mediterranean. That would involve an agreement with Assad. They also e talk about capturing Idlib from the Islamists some day.

Liberating Idlib from the Islamists would be good, provided that the inhabitants are happy with it. That is easier said than done. Who would rule Idlib instead? The inhabitants probably don't want that to be Assad, and they (being Arabs) may not be comfortable with Kurds today (though they were content enough with Salah ad-Din).