People saying that I Am was considered a let down after It Was Written need only hear the song "Rise and Fall" from Nas himself. In it, he gives a break down of all the ups and downs of his career at that time. He talks about the frustration of having people say his second album was a sophmore jinx, then hitting the studio and changing the game again and being on the top of the charts. Showing he himself, felt I Am got a warmer reception than IWW.
People throw shade on I Am now because the narrative for some time was nas couldn't top illmatic but overtime IWW status has improved in hindsight and it was probably most of his base's first introduction to him so now the shade over the years has shifted to I Am because the narrative of nas having gone through a prolonged period of "falling off" still lingers.
In reality, IWW and I Am are both largely successful albums quality wise and commercially and under any circumstance any other artist who dropped these albums would not have these projects viewed as disappointments in their discography