It's slim-down time for Wall Street firms facing curbs on some of their businesses under the financial-regulation overhaul passed by Congress last month. But restructuring or getting rid of those operations is likely to have little effect on the bottom line.

From Bank of America Corp. to Goldman Sachs Group Inc. to Morgan Stanley, executives are deciding the fate of their proprietary trading and private-equity activities. The Dodd-Frank law restricts firms from trading exclusively for their own accounts, while limiting their...