Would Bulgaria join the Allies after a breakthrough?

Would Bulgaria still have joined the Central Powers?

Probably not; a Russian-Greek victory at Constantinople could make an Allied Orthodox coalition more attractive.

The answer treats religion as a short-term mobilizing force rather than a permanent cure for Balkan rivalries. Shared interest in securing Constantinople could bring Bulgaria into the Allied camp during the war. Bulgarian alignment would also make it harder for Germany and Austria-Hungary to reach the Ottoman theater overland. Territorial and ecclesiastical disputes would likely return after victory, so this is a wartime coalition rather than a prediction of permanent Balkan unity.