Ever since the story broke that the U.S. Air Force was considering the purchase of new F-15 airframes, dubbed the F-15X, pundits and analysts alike have debated why the branch would be willing to devote billions of dollars to procure a fourth-generation aircraft in the era of stealth-capable fifth-generation platforms. Put simply, the question has consistently been, “Why buy new F-15s when we could be buying more F-35s?”

U.S. Air Force Chief of Staff General David Goldfein recently answered a number of questions about the F-35, F-15X, and the future of the force for Defense News. Some of those responses even perturbed Russia enough to get them to issue the general a threat by name via Twitter. But when it comes down to why he thinks the F-15X is a good idea, it’s really just a matter of boring old dollars and cents.

According to Goldfein, America needs to carry four fighter platforms over into combat operations throughout the 2030s: the F-35 Joint Strike Fighter, the A-10 Thunderbolt II, the F-16 Fighting Falcon, and the F-15. The only problem is, America’s F-15Cs likely won’t survive long enough to meet that requirement.

“It’s structural, right?” Goldfein said. “It’s an old airplane and getting older. It’s performed brilliantly, but the cost curves into a point where you’re spending so much money. I don’t know if you’ve ever driven an old car that dies in thousand-dollar increments. I have. It’s really painful.”

Some estimates have suggested that the cost of replacing America’s dated F-15Cs (all of which are more than 30 years old) with the F-15X could actually be recouped through savings in canceled updates planned for the older fighters, combined with a significantly reduced operating cost. The F-15X—which benefits from more than $5 billion in updates paid for by American allies still purchasing new F-15s, such as Saudi Arabia and Qatar—would also offer a series of significant improvements over America’s existing F-15s, including updated avionics and radar.

But as Goldfein put it, the need for new F-15s really comes from the need to procure new jets at a faster rate than the the F-35 can be produced.

“First, you’re not going find a stronger proponent for the F-35 than the team here. As we look to the program of record, that program of record stands for 1,763 airplanes, and so we’re going to keep the F-35 program on track,” Goldfein said. “But here’s our challenge: To build the Air Force we need, we need capacity. And our analysis shows that needs to be 72 fighters per year to be able to drive average age down, to be able to recapitalize in the right mix [according to] the analysis we did on the fight against a peer competitor.”

Throughout the 2030s, Goldfein sees the F-35 as the “quarterback” in a “penetrating joint team” made up of multiple air and ground assets that would deliver crippling blows to an enemy’s military apparatus. The F-35 utilizes its data fusion capabilities to see and relay everything that is going on throughout the battle space, helping to coordinate the actions of drones, fourth-generation fighters, stealth bombers, and even cyber warfare assets.

“If they ever do see an F-35, which is highly unlikely, it will never be alone,” said Goldfein. “We’re going be there in space—we’ve been there for a while; we’re going to be there with high altitude, ISR; we’re going be there with the B-21. It’s about to wreak havoc, and there’s nothing they can do about it. And there’s one weapon system that we’ve designed to quarterback, to call the audibles inside enemy airspace, and that’s the F-35.”

Goldfein contends that the F-15X makes sense, provided it isn’t allowed to cut into F-35 procurement funding. Because fourth- and fifth-generation fighters are going to be required to operate together through the end of the 2030s (at minimum), it’s important to properly leverage the capabilities the U.S. Air Force can bring to bear. For instance, Goldfein contends that a single fourth-generation fighter teamed up with a single F-35 can offer the force-on-force capabilities of three fighter platforms, though the general declined to specify what led to that assertion.

“They complement each other. One plus one, fourth-gen plus fifth-gen, actually equals three [fighters] if they’re used correctly because they each make each other better.”

Put simply, the general acknowledges that the future of the force is indeed the F-35, with a total of 1,763 of the fighter on order, but future numbers don’t offer combat capabilities today or tomorrow, and that’s where the F-15X comes in. As the general puts it, the Air Force is trying to bring the average age of its aircraft down from 28 years (where it currently sits) to around 15. “To do that, you’ve got to buy 72 aircraft a year. If we had the money, those would be 72 F-35s, but we’ve got to look at this from a cross-business case. The F-15 will never be the F-35. But I need an asset.”