Goldman Sachs (NYSE:GS) has had it with the discounters. The investment banker is downgrading its outlook on the discount-brokerage industry, on fears that margins and fee revenue will come under pressure in a low-interest-rate environment.

Goldman Sachs is knocking TD AMERITRADE (NASDAQ:AMTD) down from "buy" to "neutral." It's also lowering its price target on TD AMERITRADE, E*Trade (NASDAQ:ETFC), optionsXpress Holdings (NASDAQ:OXPS), Charles Schwab (NASDAQ:SCHW), and TradeStation (NASDAQ:TRAD).

At least fans of the discounters can take heart in a recent Aite Group study. The financial-services research and advisory firm found that discounters have taken in 25% more assets from full-service brokers over the past two years than the other way around.

It's fine that Goldman is souring on an industry that it competes against, and there's merit to its claims of a lousy environment for interest rates. Discounters have had to waive fees to keep their money market funds competitive, and still they offer a crummy value proposition.

E*Trade's flagship Complete Savings Account yielded a healthy 3.01% when the year began. Now, free-falling rates have pushed that yield all the way down to 0.50%, with its Max-Rate checking serving up an uninspiring 0.30% annualized payout.

There's not a whole lot the E*Trade Baby can say when six-month and 12-month CD rates are 0.05% and 0.30%, respectively. Savers will be hard-pressed to find substantially better rates elsewhere these days, but online banking still looks even less attractive as a result.

As long as discounters continue to nibble away at their full-service peers, it's hard to bet against the industry the way Goldman suggests. However, Goldman is probably spot-on when it comes to the near-term challenges.

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Longtime Fool contributor Rick Munarriz has been trading exclusively through discount brokers since 1990, but he owns no shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Foolhas a disclosure policy.