Should private equity succeed in fulfilling a longtime “dream” of tapping into the trillions of dollars held in 401(k)s and other defined-contribution retirement plans — which workers pay into, sometimes with an employer match — one executive told the Financial Times it could potentially double demand for the industry’s various funds. But advocates warn that private equity could endanger retirement savings in the same way the industry has imperiled public pension plans — by charging exorbitant fees and funneling money into risky and opaque investments.