Odd that the question marks disappeared but anyway it's clear that BA did not dispose of its classics because of any safety issue regarding the blocked exit.
The fact is that the type of mission that BA would have tasked a B747-200 with at the time they were ordered and delivered is now covered more efficiently by newer aircraft.
It's also a mistake to assume that the B742s were directly replaced by B744s. The B777 and B767 are also used on missions now that were the sole province of the B747 classics ten or fifteen years ago.