YPG is one of the numerous Peshmerga factions. There are other larger factions, which are ready to fight the ISIS in its core heartlands. And cities such as Mosul historically belonged to the Kurdistan, although the current population is mostly composed of Sunni Arabs. The Kurds will be ready to liberate Mosul from the ISIS, and if they can do that, it will be a devastating blow for the ISIS.
My other concern is the effectiveness of peshmerga fighters. Frankly, they are not very good. Their victories have been exaggerated, IMO. When I looked into their battlefield tactics their mystique diminished greatly. They use a "rolling attack" almost 100% of the time. That technique must have shocked DASEH, who are used to the enemy running away in disarray. But it is a technique that can be resisted effectively if you know its coming. So repeating the recent victories will get harder and, once the enemy has learned, will no longer work.
A rolling attack requires brave fighters and momentum. Basically they keep moving forward at all costs. It is terrifying to see such an attack coming at you, but if you keep your cool and halt the attack it tends to fall apart. At that point you sally from your positions and slaughter the attackers as they try to escape. I think the peshmerga are vulnerable to such a counterattack. Once the attack starts they don't communicate much. If their line collapses anywhere then it's chaos. ...