Once the Janisaries were inside the walls, Constantinople was doomed
I disagree ,for several reasons.
Firstly , this was not the first time that islamic armies had threatened the walls of constantinople. there were in fact over 20 seiges of the city (and at least one sacking during the third crusade) of which none had successfuly breeched the walls. The mere prescence of the army , however greatly it outnumbered the defenders , does not presume defeat.
secondly ,the ottoman army, however vast ,would have been useless had it not been for a significant amount of innovation on behalf of mehmet II. the use of oversized seige cannons to cripple the vast walls was a turning point in history , effectively ending the days of the castle-fortress. But it was an innovation and there was no guarrentee it would work. If the cannons had cracked (as cannons in this period did regularly) then mehmet would have had significant difficulty replacing them. As for using them against an inner line of defence, this would have been tricky ,the inner walls were surrounded by development , clearing space and moving the cannon into position in such an environment would have bee unteniable. However the establiahment of a defence line of this nature was unlikely , the byzantines had too few troops.
as well as this there was the role of the ottoman navy. Thier inability to control the waters around the city was a key factor in the early stages of the battle. with a allied fleet in the goldern horn ready to sally against a amphibious operation , the Byzantines could afford to move the few troops they had to the landward walls. to counter this Mehmet had galleys moved OVERLAND and into the golden horn behind the allied fleet in secret , forcing them to flee. had the allies been able to react in time and stop this smaller fleet from compromising them , or had mehmet not dreamed it up , then it is arguable that the final assault in the Theodosian walls ,already a bloodbath, would have turned into a rout.
Thirdly , there was the untimely injury of the Geonosian commander
Giovanni Giustiniani on the walls. His loss caused the Essential Geonosian mercanaries to retreat to the harbour ,loosing the brilliant defensive mind of Giovanni , and the loss of the best trained ,best equipped men on the walls. i believe that had he remained in command ,backed up by reinforcements from the walls ,then he would have turned the killing grounds between the inner and outer walls into a slaughter. Venitian troops at the battle used small hand cannon as we do shotguns , blowing away 20 or more ottomans at a time...
with the loss of significant chunks of his infantry during several assaults on the walls ,and particularly during this last charge , it is arguable that the already questionable loyalty of mehmets troops would have been pushed too far , resulting in mutiny or the empowerment of several elements of the ottoman command that favoured retreat.
On the other hand ,whilst the fall of constantinople in 1453 was not inevitable , i suspect its eventual collapse was. the byzantine empire had been in retreat for centureis leading up to 1453 and without significant aid from the west , im almost certain that another siege would have toppled it before the end of the 15th century
(guess who got a book about this last christmas

)
Sir Scott
-Stood on the walls of Justinian