The 4th crusade forever sealed the fate of Byzantium, sure it briefly came back and reclaimed the city but it would never be as grand, prestigious, or beautiful again until after Ottoman conquest. The reason Venician troops were able to enter the city in the first place was because apparently they paid off some of the garrison to open a gate for them, the Theodosian walls were all but invincible to contemporary siege weapons which is why the city held out so long against Mehmet who needed cannon to bring them down. Point being Venice didn't sack Constantinople through brute force alone, it was conspiracy and subterfuge that defeated the Greeks that day. If the 4th crusade didn't end with the sacking of Constantinople even if it was ultimately a failure could've bought the empire time to gather forces to repel the Ottomans, strong emperors could've won the loyalty of the Balkan nations and possibly stopped the European part of the empire from falling