If you can steal the plans for an F-35, you won't need to spend as much money to develop your own. You can just build one and test it to see how it compares to anything your own engineers came up with along with the American-made F-35 test results you stole too. Then you can spend that money on trying to fix anything that might be wrong with the American design.

Sure, with enough time and money, your guys might have been able to come up with a better design but if you can steal at least something to build off of, why not? It's cheaper to pay some kid hackers to steal it than design it.