Sure, but how many sieges where frontal assaults were used ended well for the attackers? I've had a quick look, and while numbers are rarely available, the few I have seen seem to indicate that assaulting with anything less than 5 to 1 is often doomed to fail against moderately fortified defenses. Svetigrad (Albania) for example was outnumbered by the Ottomans 40 to 1 and held out longer than Constantinople, though admittedly they also had a good number of troops outside the walls lead by Skanderbeg harassing the Ottomans that brings down how outnumbered they were to around 6 to 1. Or the siege of Krujë, outnumbered by ~10 to 1 like Constantinople was, but again lasted longer than Constantinople and forced the Ottomans to lift the siege 6 months later.
So lasting 6 weeks being outnumbered 10 to 1 and inflicting an unknown number of causalities doesn't appear to be very impressive given the walls. Hence I stand by my earlier assertion, the walls may have been great, but the defenders were not. A boost to local fort defensiveness isn't warranted, and if the fort level gets bumped up to mimic the walls, it should also get a heavy penalty to defensiveness to mimic the people.
I'm all for ahistorical outcomes, and I've seen Byzantium survive and prosper without my intervention. I don't really see a need to increase the possibility of it occurring, and I don't really see how bumping up Constantinople's defenses will even change anything. All it will really do is drag out the siege longer, when it's already ahistorically long nearly 100% of the time, and then give the captors a shinier fort in their new capital. Which I'm fine with really (I never said anything about it being OP). I'm just not fine with boosting Constantinople's defenses just to try and make Byzantium last longer before capitulating.