Could be that Schwinn was tired of getting more possible legal litigation over new names that they could just go ahead and use the Spitfire name they already owned, and stay busy selling bikes.
When the Spitfire came out in 78, it sold like hotcakes, every one in my middle school and high school wanted one.
I got my first brand new bike as a black Schwinn Spitfire from Aviation Cycles in Manhattan Beach . They didn’t have the black in stock, I then impatiently wanted a blue or red one but my mom made me wait for the black one. It got stolen at school then I got a black 5 speed Spitfire from Circle Cycle in Torrance.
The Schwinn beach cruiser popularity was short lived imho as the 26 bmx bikes came and also died with the arrival of 24 bmx.