50,000 people would still put it at least into the top five in Europe, probably the top 3, and it is wealthy merely by virtue of the amount of gold and jewels stored in it's churches and palaces.
Not by 1448, on either score.
Look at the Italian cities like Genoa and Milan and Florence and Venice. Look at Granada and London and Paris.
And those are just the ones coming to mind. 50,000 as a high estimate barely makes it into this category.
As for the gold and jewels in it's churches and palaces: What gold and jewels?
The Hagia Sophia itself is in poor condition, how do you think there are gold and jewels in abundance in that situation?