Debate continues to rage among pundits and internet warriors alike about the value (or lack thereof) in the Air Force ordering 80 brand new F-15X air frames in the coming years. Some contend that the billions of dollars being spent on these new “old” aircraft would be better invested by expanding orders of the more technologically advanced F-35. Those in the F-15X camp point out that F-15s are expected to remain a part of the Air Force’s military strategy for years to come, and claim that, despite lacking in stealth and data fusion technology, the F-15 still has some tricks up its sleeve that the F-35 lacks.

Despite a laundry list of ongoing issues with the F-35, including ongoing concerns about the Air Force variant’s close air support gun and only 60 percent of delivered jets actually being able to fly, the F-35 is the future of American air power. With more than a thousand F-35s on order and early reports indicating that the F-35 once again dominated Red Flag dog fighting exercises against other American fighters earlier this year, it’s safe to say that even a troubled fifth-generation platform offers a significant advantage over capable fourth-generation fighters like the F-15, especially compared to America’s existing fleet of F-15Cs and Ds—the youngest of which is more than 33 years old.

However, the F-15X would reportedly boast significantly lower operating costs when compared to the F-35 or even America’s existing F-15s. It also has a much higher top speed than the F-35, and can carry more weapons, including forthcoming hypersonic missiles the F-35 won’t be able to carry without compromising its stealth profile. The fact that F-15Xs could be purchased at a set price of $50 to $75 million each ruffles even more feathers, especially when you consider that F-35 unit prices are expected to drop to $80 million each within the next few years.

But the truth of the matter is, comparing the F-15X to the F-35 is a fool’s errand; they have different operational obligations and the U.S. isn’t choosing between the F-15X and the F-35 (in fact, both Pentagon and Lockheed Martin officials have stated plainly that F-35 orders are not being affected by F-15X purchases). The real debate should be between the F-15X and America’s existing stable of F-15s.

Last year, it was revealed that a plan to upgrade just the electronic warfare equipment in America’s existing F-15s would cost around $3.4 billion—more than half of the $6.4 billion allocated for the purchase of 80 new F-15Xs, disregarding startup expenses. That upgrade was significantly slashed, and it remains unclear how many F-15s have seen the new equipment, but it’s also far from the only issue these old jets have run into recently.

According to reports from last year, when the F-15’s lifespan was the subject of a great deal of debate, the common figures bounced around were between $30 and $40 million per fighter to keep the Air Force’s existing F-15s airborne through the 2020s. That means each jet’s service live extension would cost half the price of a new one alone, before you consider the reduced cost of operating the F-15X when compared to its older siblings, and without including significant improvements in onboard systems.

“On any aircraft, particularly when they get older, when you look at them, you have to determine how much is it going to cost and how invasive is it to do modifications that we would need to do to extend its life and to keep it going forward.” Lt. Gen. Arnold Bunch, the Air Force’s top uniformed acquisition official, told Defense News about the F-15 last year.

“There comes a point with an older aircraft, that if I have to go in and I have to basically take major components out, and it costs me so much money and I’m not gaining service life, well it may not be worth it.”

Put simply, the older an aircraft is, the more money it costs to keep them flying.

The F-15X benefits from years’ worth of research and development—around five billion dollars’ worth—thanks to its continued production and exportation to American allies in nations like Saudi Arabia and Qatar, enabling it to bring more advanced combat and flight systems into American service without the need for any significant investment to make the upgrades. With a mind-boggling proposed 20,000-flight-hour lifespan, the F-15X would also remain America’s fastest fighter jet in service, while boasting some of the best fourth-generation hardware on the planet. That may not make it a match for the F-35 in many situations, but it would make it more than a match for most of the fighters being fielded by America’s opponents. That is what the F-15 was built for, after all: air superiority.

It’s important to note, however, that purchasing the F-15X does not mean the Air Force will toss out their old F-15s, and the high price of keeping these old birds flying is likely to continue to drain Uncle Sam’s coffers for years to come. But with all the arguments about the F-35 and the F-15X, it pays to remember that these aren’t the only two fighters America has in the air. We never had to pick just one before, and the Pentagon already plans to begin fielding a new air superiority fighter sometime in the 2020s, because that was truly never what the F-35 was built to do.

Purchasing the F-15X, it could be argued, is more an acknowledgement that the F-22 program should never have been canceled than it is about the F-35. America needs air superiority fighters, and while the F-35 may be good in a fight, it may not always be the best choice for every fight. Especially ones where money is a primary concern.