Nevertheless, “this is a period of time when I’m feeling quite optimistic about our positioning,” Mr. Blankfein said. “I’m also feeling quite good about the external environment.”

He added that there was no personal reason for his departure.

“Could it have been earlier?” he said. “Could it be later? I’m not tired. I’m well. I’m not out of gas.” But he added, “David is ripe and ready and the right guy.”

Already, Mr. Solomon has pushed for changes to Goldman’s business. He has introduced smarter technology in stock trading and investment management. He has moved salespeople from corporate-trading desks into the investment-banking division to help streamline interactions with clients. He is expanding Goldman’s nascent consumer bank, which is called Marcus, into new areas.

To some analysts and other close watchers of Goldman, the shift should have come sooner.

“Lloyd Blankfein’s legacy will be defined by how Goldman performs over the next couple of years after he’s gone,” said Mike Mayo, a banking analyst at Wells Fargo who has covered Goldman for 15 years. The firm’s growth plan, he added, “would fall into the category better late than never.”

During Mr. Blankfein’s first full year running Goldman Sachs, the firm brought in close to $1 billion per week in revenue, thanks largely to its stock and bond trading. Then came the financial crisis that hobbled the banking industry, followed by a return to robust profits in 2009. Those were later challenged by a raft of new regulations that upset its legacy business model.

“There were opportunities in my time that I may have missed, and there were traps that I caught,” Mr. Blankfein said. “At the end of the day, the firm during my tenure faced existential risk in terms of the first half of the financial crisis and extraordinary reputational risk, which came about partially because we navigated the existential risk as well as we did.”

The firm also faced a raft of criticism for profiting from the financial crisis at the expense of some clients. It incurred a $550 million fine from the Securities and Exchange Commission for its actions.