"If you're a fiduciary to a $1 billion-plus plan, there's no way you should be offering retail-class shares of mutual funds," he said.

The Supreme Court's decision overturned the 9th Circuit Court of Appeals. In 2007, participants in utility-giant Edison International's 401(k) plan alleged millions in losses to retirement investments because fiduciaries offered six retail-share mutual funds, when materially equivalent and cheaper institutional shares existed.

The U.S. District Court for the Central District of California found Edison breached its fiduciary obligation relative to three of the funds added to the investment lineup in 2002.

In a three-day bench trial, Judge Stefan Wilson ruled Edison offered no "credible reason why the plan fiduciaries chose the retail share classes," and that no evidence had been presented that Edison "even considered or evaluated the different share classes when the funds were added to the plan," according to court documents.

But claims against the other three retail mutual funds, added in 1999, were dismissed as untimely. ERISA's six-year statute of limitations bars participants' claims after that period, ruled the lower court. The 9th Circuit upheld that decision.

Both lower courts were wrong, the Supreme Court said.

"ERISA's fiduciary duty is derived from the common law of trusts, which provides that a trustee has a continuing duty–separate and apart from the duty to exercise prudence in selecting investments at the outset—to monitor, and remove imprudent, trust investments," wrote Justice Stephen Breyer, who delivered the opinion for the court.

A statement issued from Southern California Edison after the ruling said, "the companies' and plan fiduciaries' efforts to act in the best interests of plan participants are reflected in the numerous rulings in our favor in the class action challenge to our 401(k) plan.

"The courts have ruled in our favor on every claim, with the sole exception of six mutual funds that were removed from the plan's investment lineup years ago. And with respect to those six mutual funds, the courts have found that we satisfied our duty of loyalty to plan participants," according to the statement.

But the court was clear on fiduciaries ongoing responsibility to monitor investments, and Schlichter thinks that means a clear fate for retail-class shares in big plans going forward.

"Warren Buffet doesn't pay retail, and neither should participants in a multi-billion dollar plan," said Schlichter. "When you prudently monitor plans, there's no way anyone can rightfully conclude that participants should be paying higher fees."

In a previous interview with BenefitsPro, Schlichter refuted the idea that a favorable decision for his clients in the Tibble case would spur a rash of new claims against investment decisions made years ago, outside ERISA's statute of limitations.

But he does think that the High Court's ruling should put all plans and advisors on notice. "Many institutional funds have rates for as little as $1 million in investments," he explained. "There is obviously no question Edison could have obtained those rates."

At the time of the original claim, Edison's plan was said to hold about $4 billion in assets. How smaller plans are affected by the decision remains to be seen, or whether or not the mutual fund industry will make institutional products more readily available to smaller plans.

But industry groups that came out in support of Edison pointed out in an amicus brief to the Supreme Court that there can be good reasons for a sponsor to use a retail-class mutual fund, according to ERISA attorney Marcia Wagner, principal of the Wagner Group.

"Revenue sharing generated by these funds can support a higher level of services that otherwise would have to be paid for directly by participants or the plan sponsor," wrote Wagner in response to the Supreme Court's ruling.

While the Supreme Court was clear in plan fiduciaries' ongoing obligation to monitor plans, it stopped short in defining just what the monitoring process should be, remanding that question back to the 9th Circuit.

Schlichter can't imagine the lower courts will rule in anyway unfavorable to the class of current and former Edison employees.

"They've already ruled that there was a breach with respect to the three funds added in 2002. The claims that they dismissed were against funds that had the exact same defect," he said.

In effect, the only way lower courts could now rule against the three funds added in 1999 would be to overrule the findings for the funds added in 2002—an unimaginable scenario to Schlichter.

When the lower courts do reconsider the case, is it possible they could rule that Edison adequately satisfied a duty to monitor with respect to the three funds added in 1999?

Wagner called the chances of that "very low."

But she did say there is a possibility the 9th Circuit could issue a new decision on procedural grounds.

In reconsidering the case, the lower court will hear Edison's argument that the plaintiffs did not raise the argument that fiduciaries' failure to monitor the plan was a new breach until the case reached the Supreme Court.

"It is possible, albeit somewhat unlikely, that there may never be guidance from Tibble on the scope of the duty to monitor," said Wagner.

Schlichter said the plaintiffs in Tibble have already adequately refuted that part of Edison's defense.

When the case does move forward, which Schlichter expects to happen reasonably soon, given the age of the case, he's fully confident his clients will come out victorious.