Large numbers of cheap, powerful, reliable aircraft is a hard combination to over come.
This.
Defense strategists have to decide which path to take...high tech, expensive, aircraft with huge capabilities...or, cheaper, agile, fighters that you can mass produce.
The T-38 and F-5 are not the same aircraft. The t-38 is a twin engine trainer capable of exceeding the speed of sound. I have 120 hours in it.
The F-5 has, iirc, a single engine. It is designed as a combat aircraft. It was used to simulate enemy fighters at combat tactics schools.