Still, about two-thirds of Fidelity's clients remain "do it yourselfers," according to the company's own analysis of 21,000 DC plans and 13 million participants who use Fidelity. 

Also, more than half of those workers have not engaged with their plans since 2012; three of four participants told Fidelity they lack the expertise and discipline to manage their own money. 

Sangeeta Moorjani, vice president of Fidelity's Professional Services Group, thinks that's why employers are now acting. 

"We continue to see a significant demand for workplace managed accounts as more companies seek to offer professional management for their employees and benefit from having a provider assume the fiduciary responsibility for those investment," she said.

Most agree managed accounts will only grow. 

Cerulli Associates, a Boston-based research firm, is expecting the managed account market to hit $6.7 trillion by 2017. 

As is, 401(k) managed accounts alone hold about $108 billion in assets, according to Cerulli. The Plan Sponsor Council of America says that 36 percent of employers offered managed accounts in 2012, up from 25 percent in 2005.

But managed accounts have drawn fire, too.

Last summer, a Government Accountability Office investigation of eight unnamed managed account providers found a wide discrepancy in the fees they charge—between $8 and $100 on every $10,000 invested.

Participants in managed accounts can expect to see "improved diversification and experience higher savings rates compared to those not enrolled in the service; however, these advantages can be offset by paying additional fees over time," the GAO noted.