What was made can be unmade.

JPMorgan Chase and Wells Fargo may have venerable names, but they and the pseudo-venerable Citigroup and Bank of America are all products of countless mergers and agglomerations.

There is no rule of markets that requires a financial system dominated by four cobbled-together, lumbering behemoths.

Lawmakers and regulators have failed to remake our system with smaller, safer institutions. What about investors?

Big bank stocks have been persistently weak, making breakups that seemed politically impossible no longer unthinkable.

Bank of America’s recent quarterly earnings were so weak that investors and commentators wondered whether the bank should sell off Merrill Lynch, the investment bank for which it foolishly overpaid at the height of the crisis. Bank of America trades at half of its book value (the stated value of its assets minus its liabilities), an indication that investors view its asset quality and prospects just a notch below abominable, as Jonathan Weil of Bloomberg News pointed out last week.

For Bank of America, the question is whether it will have to raise capital. Selling shares at such depressed prices would be costly. Regulators won’t push for it. They just gave stress tests to the biggest banks and merely restricted the bank from paying out a dividend. The logical solution is that Bank of America shed business lines in a bid to improve its prospects in the eyes of Wall Street.

Citigroup’s stock, revenue and earnings have lagged for a decade.

“Look, if you can’t compete in the major leagues for over a decade, it’s time to go back to the minors,” said the always outspoken Mike Mayo, an analyst with CLSA. His chronicle of ruffling bank management feathers, “Exile on Wall Street” (Wiley), will be published in the fall.

JPMorgan Chase is as well managed as any gargantuan bank can be. But if you look at its businesses, it’s hard to see any area where it is clearly the best, something even its own executives concede. Not in credit cards, where the premier name is American Express. Not in money management, where you might offer up T. Rowe Price. Investment banking—Goldman Sachs (the last quarter notwithstanding). Back-office transactions, State Street.

Yet even JPMorgan is merely trading at book value. Put another way, the market regards the value that JPMorgan provides as a financial services conglomerate as zilch. How well do all of JPMorgan’s divisions work together? In presentations to investors, JPMorgan executives show how much revenue they gain from existing clients. But these measures are hardly unbiased. Executives have an incentive to defend their empires. Who is to say that a certain division of JPMorgan wouldn’t have won that business anyway? And nobody measures how much a bank loses through conflicts of interest.

Even in the face of investor pressure, there are forces that would hold bank breakups back. Mainly pay.

“The biggest motivation for not breaking up is that top managers would earn less,” Mayo said. “That is part of the breakdown in the owner/manager relationship. That’s a breakdown in capitalism.”

Institutional investors—the major owners of the banks—are passive and conflicted. They don’t like to go public with complaints. They have extensive business ties with the banks. The few hedge fund activist investors who aren’t cowed would most likely balk at taking on such an enormous target.

Also, there are reasons to think that smaller banks wouldn’t necessarily make the system safer. A wave of small bank failures can have systemic effects, as was the case in the Great Depression. Focused companies like Washington Mutual and Bear Stearns failed in the recent crisis, worsening it.

Making a nuanced argument, John Hempton, a blogger, investor and former regulator in Australia, says that it’s better for shareholders—and societies—to have large banks with lots of market power. That makes them more profitable and leads them to take less risk, making them safer and more enticing for investors.

Another oft-trotted-out argument against breakups: The United States needs global banks to service its giant, multinational corporations and to preserve our position in world markets.

Color me unconvinced. When a giant corporation wants to do a major bond offering or a big company goes public, the banks, despite their size, don’t want to shoulder all the risk themselves, preferring to share the responsibility.

If the stocks continue to lag for quarters upon years, these arguments will seem less convincing, while institutional reluctance will begin to erode.

Investors don’t care about size, they care about performance. It’s undeniable that smaller banks are easier to manage. And they are easier for regulators to unwind—and therefore less terrifying to trading partners—when they fail.

One of the most remarkable aspects of the debate about overhauling the financial system after the great crisis was the absence of serious contemplation of breaking up the largest banks.

It’s not a perfect solution. Banks responding to investor pressure would react haphazardly. But it’s a good start.