I shall stick up for Bishop Ussher by saying it is easy in hindsight to use modern knowlege to point and laugh at somebody from the distant past and from a very different time.
I am sure to physicists 400 years from now, some of our scientific hypotheses will look utterly laughable.
Bishop Ussher made a commendable attempt using accepted methods of his time. He was not a stupid man, but was part of an intellectual tradition which tried to establish answers about the natural world using conventional method of scholarship of the day.
The problem is this: the fact the anyone would believe his estimate today, in light of scientific advances, highlights who the real idiots among us are.
Sidebar: I recall that the ancient Hindus thought the Earth was around 4 billion years old, so at least they were on the right track of conceptualizing an ancient earth.