It's not about "viability". It's about power. Assad will crush enough people to re-establish his control over most of the country, if not all of it. The Sunnis may not like him, but if the choice is going back to Assad-controlled Syria, living forever in tents as refugees or risking their lives to try to get to Europe, plenty of them will go back to Assad-controlled Syria. And at some point neighboring countries will kick them out anyway.
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*