There is very likely no line that would apply uniformly between people or even the same person in different states of stress and health, regardless of the exact criteria you are optimizing for. Some people find even coffee to be too much at times. But at the end of the day amphetamines have some pretty scary and dangerous long term documented effects especially if the person goes without sleep for like periods of time. It’s not as bad as meth (to the best of my understanding) but both are amphetamines.

"Drugs I use are fine, but I don't use are evil and horrible."

When taken as directed I'd be inclined to agree, but there is definitely a non-negligible likelihood of individuals misusing it and forming a dependence as a result, more immediately so than caffeine.

Also coca and khat leaves in different parts of the world.

Ha - let's freely dispense amphetamines to all, it'll surely help the economy!

Edit: We're talking about amphetamine based production being non sustainable - by comparing with tea and coffee I imagine you mean that we should expect people to take amphetamines to the same extant they take other stimulants (ie: to wake up, to work longer, to focus, etc). We shouldn't expect everyone to be on stimulants, we should lower the workload.

This has been a problem for years in academia and in medecine - people taking amphetamines and cocaine to be able to compete in their field. Without talking about expectations in other fields - I've seen my fair share of cocaine and speed in kitchens and in service - I don't think it's sustainable.

If you need speed, the workload is too much for one individual. Speed is not a long term solution.