BEIRUT, Lebanon — With the Syrian government making large territorial gains in Aleppo on Monday, routing rebel fighters and sending thousands of people fleeing for their lives, President Bashar al-Assad is starting to look as if he may survive the uprising, even in the estimation of some of his staunchest opponents. Yet, Mr. Assad’s victory, if he should achieve it, may well be Pyrrhic: He would rule over an economic wasteland hampered by a low-level insurgency with no end in sight, diplomats and experts in the Middle East and elsewhere say.