Sprint (S) and T-Mobile USA (TMUS) seem to be plunging ahead with a possible merger despite initial skepticism from antitrust regulators.

The latest merger leaks, including a possible price of $40 per T-Mobile share, were accompanied by a rash of upbeat stories about a softening of regulatory opposition.

But antitrust experts say the still theoretical deal still faces enormous antitrust hurdles. Both the U.S. Justice Department’s antitrust division and the independent Federal Communications Commission must approve a combination.

The possible deal would combine the third-and fourth-largest U.S. wireless carriers, which could create a more powerful competitor to top carriers AT&T Wireless (T) and Verizon (VZ), according to proponents of the deal.

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“It is an argument,” says Harry First, a law professor and antitrust expert at New York University. “But not an unfamiliar one to regulators.”

The problem for the companies is that the historical record has many examples of competition withering with just three big players, he says. That includes the history of the wireless market a decade ago, before T-Mobile emerged as a fourth national carrier.

“In an industry this concentrated, normally going from four to three will not make it more competitive,” First says. “The incentives are to be more cooperative than competitive.”

That was the rational when regulators blocked AT&T from acquiring T-Mobile in 2011. In wireless, where plans are national in scope and prices are public, it’s all too easy for three big players to coordinate and keep prices in line, not to mention minimizing capital spending to improve network quality.

Combining the number three and four players is somewhat less troubling, says Christopher Yoo, a law professor at the University of Pennsylvania. Ultimately, authorities will review the impact on markets down to a city-by-city analysis, he says.

This time around, the companies are talking about how much more competitive they could be if they were allowed to merge. Japanese billionaire Masayoshi Son, who bought Sprint for $22 billion last year, says he needs greater scale to take on Verizon and AT&T.

“We can start a small fight, but it does not scale, it does not last, it’s not sustainable,” he said in a speech in Washington, D.C., in March.

That came after William Baer, the assistant attorney general for antitrust, told The New York Times: “It’s going to be hard for someone to make a persuasive case that reducing four firms to three is actually going to improve competition.”