Why, exactly?
Yes, I'm well aware historically Byzantium was on life support in 1444. But a skilled player with alot of luck can turn around anything in these games, its just the way it is. I've done a quasi-WC with Granada. Fortunately for you, you dont have to play Byzantium, and you can even set your start date to 1453 whenever you wish. Furthermore, I'll add that I'm certain AI Byzantium will fall in short other every time, as it should, so none of this impacts your games. Finally, I actually love the Ottomans too, and I'll certainly play many games as them as well. This is really not an either-or situation, and setting the start to 1453 achieves nothing except spite players who enjoy a little in-extremis alternate history; it made a huge mess last time, I dont see why we'd want a repeat of that. 1444 is a pretty good start point, giving us 50 years before the new world gameplay kicks in.
And hey, it's a moddable game. You can make Byzantium fall in 1439 in your game files if you wish. Just as I'll be able to restore the EU3 cores if they get nerfed any further.