Quitting amphetamines after a long time of use would definitely make your brain go blank for a while.
Amphetamines aren't used for creativity. They more accurately could be characterized as "reducing the activation energy for entering (and resuming) the focused state needed for a task".
quitting caffeine by contrast, will cause terrible headaches and a shitty mood for a at least half a week.
That seems like a very doubtful claim. To my knowledge, for example, antibiotics are pretty much 100% safe (unless you have an allergy). Are you indeed comparing amphetamines to all other prescription drugs? Or perhaps only to prescription drugs with abuse potential?