What if Constantinople had never fallen to the Turks?

[Prussian to the Iron]

So basically you are saying it was a replay of the fall of the Western Roman Empire? Internal conflicts and external threats.

well, rome kinda had a decline starting with bad emperors, and ending due to bad timing with the invading goths, huns, etc.

whereas byzantium slowly wore itself down financially and politically due to internal squabbles less than bad emperors (though these of course weren't uncommon, as in any state/country/empire/nation) and trying to overstretch themselves. of course the crusaders sacking of constantinople was a huge loss.