I don't have a source, but it actually is quite likely, given the legacy of the Balkan wars (especially the 1st) and Bulgaria's territorial ambitions. Hell, even without the Bulgarian flank secure, you've still got a potentially dangerous Greece, and the entente that could attempt an attack via Bulgaria without Bulgaria officially entering the war (much like the Salonika landings). As has been said, capturing Constantinople would have been a massive prestige boost, in addition to probably bringing the Ottomans to the peace table, and securing a black sea passage to Russia (if the entente could have de-mined the straits too..).
Given entente naval superiority, it also makes sense (I think) to keep your best men somewhere where they could act defensively quickly against a powerful aggressor. Without Bulgaria, there is no land link to the rest of the Central powers either, thus no way to receive any kind of reinforcement from the other powers, no matter how marginal, if their had been an attack on Constantinople.