"The business model is broken and 50 per cent of the people in our world who are in trading are stuck right now," Maloney said in an interview. "For every 10 of them there's going to be three or four left. What's the timeframe? Well, everybody I know is looking for a job - not looking for a job, looking for a career."

While the size of the US bond market ballooned by more than US$5 trillion since 2008 to US$37.8 trillion at the end of last year, trading in the debt has slumped, according to data from the Securities Industry & Financial Markets Association. Average daily turnover fell to US$809 billion last year from US$1.04 trillion in 2008.

That is partly because banks have pulled back from making markets in bonds as higher capital requirements make it less profitable. The business - where buyers and sellers are primarily matched over the telephone or through emails - has also suffered shrinking margins because of regulator-mandated price transparency and the rise of electronic trading.

Transaction costs declined after the Financial Industry Regulatory Authority introduced its bond-price reporting system, called Trace, in 2002. Wall Street bond traders lost about US$1 billion in fees in the following year, or about US$2,000 a trade, according to a study in the . The system is intended to provide transparency in an opaque market, and help prevent investors from being fleeced.

"Despite attempts to get it to be electronic, it's still a voice market," said Mark Pibl, head of research and fixed income strategy at Canaccord Genuity. "On the equity side, there's just one security whereas on the bonds of an issuer there could be five or six or more, there could be different subordinated issues. To a large degree, that complexity within the capital structure doesn't allow itself to easily be automated."

Since the credit crisis, jobs and compensation have declined as Wall Street retrenched. Total pay at the biggest banks has fallen as much as 50 per cent for high-yield and investment-grade traders and up to 25 per cent for distressed-debt traders since 2010, according to New York-based recruitment firm Options Group.

The decline of the profession matters because it has become harder for everyone to buy and sell debt as banks cut the amount of capital they are devoting to trading. Primary dealers slashed the amount of bonds they hold by 76 per cent from a record high in October 2007 to US$56 billion in March 2013, when the US Federal Reserve changed the way it reported the data.

Liquidity may only get worse when bond prices start falling as the Fed withdraws its record stimulus, which may make the market's swings more dramatic, according to Peter Tchir, head of macro strategy at Brean Capital. "Banks are less able to and willing to take risks," he said. "The real risk when rates rise is that you get heightened volatility because the nature of this business has changed."

The evidence shows that the business has lost its lustre, holding little appeal for newcomers, said Stein, 56. He said when he started out at Drexel in 1985, Wall Street was "the hottest job outside of Hollywood". That was two years before Tom Wolfe referred to bond traders as the "Masters of the Universe" in his 1987 novel that chronicled Wall Street excesses.

"When I started, I think you could count on one hand the number of people over 35," said Stein, who said he was not sure he would advise his son to follow in his footsteps. "Now I don't think being 50 in this business makes you an outlier anymore. A little grey hair is a good thing - it means you've had the opportunity to see good and bad markets."