The Fourth Crusade was indeed an abhorrent act. When was it? 1202 to 1204.
It is false to blame the fall in 1453 on something 250 years before is rather like blaming 9/11 on weakness caused by the French and Indian War.
It makes as much and more sense to blame the fall to the Ottomans on taking of the city by Michael VIII in 1261. That was 50 years closer in time to the Fall than the 4th Crusade. What had the Palaeologi done for the last 200 years? If they were not going to do anything, why not let the Latins keep it? Why not work with the Latins, or if they had to take the city, why not emulate the Comemneni to fix the weaknesses and retake the lost territories. The Comemeni did that in only 4 years.
Rather than that, the Palaeogi and their apologists blamed others for their inaction and their failings.
You misunderstand me. I wasn’t blaming the sack of Constantinople in 1204 for the fall. I was simply responding that getting stabbed in the back didn’t help matters. In fact, if you read what I wrote, I attributed the fall largely to the Romans’ inability to unite against their enemies.
However, I would point out the simple fact that actions have consequences. To use your example, it would not be remiss to use the Seven Year’s war to explain why the U.S. is an independent country—and to explain the the run of dominance experienced by the British—this history, combined with what followed, explains the widespread nature of “Anglo” culture around the world. Similarly, is would be correct to state that the Battle of Manikert in 1071 led to the Crusades. These Crusades helped peak European interest in the East. This in turn led to an increase in exploration which resulted in the discovery of the America’s and the West’s emregence as a dominant power.
Also, it is undeniable that the sack of Constantinople fragmented the Empire. This had already begun earlier with the “secession” of the Empire of Trebizond. Even after the city was retaken, the Despotate of Epiros was in conflict with the Palaelogoi. Furthermore, their involvement with the Latins helped retain Western interest in the region. This forced the regime in Constantinople to pay attention to threats from the West—often to the detriment of the territory in Asia Minor. It is debatable whether or not they should have retaken Constantinople, but to expect them not to jump at the opportunity would be unreasonable.
I would also take a good look at the effect that Michael VIII’s dalliance with the Church of Rome had on the public. I also wouldn’t compare the Palaeologoi with the Comneni too closely. The disparity in resources was significant. IMO Alexius and John Comnenus were pretty good emperors by any standards, but I would say the challenges they faced—although very serious—were not on the same level as those faced by the Palaeologan dynasty.