Well, it's not clear why it failed exactly. It might have been because it never existed before, but it seems more likely to me that it was because Harry didn't know what it was exactly. He didn't try to transmute "this molecular structure I have in my mind", he tried to transmute "a substance I know nothing about except that it cures Alzheimer's". That was probably not specific enough for the spell. (Otherwise, why not transmute a black-box device with a big red "kill Voldemort where-ever he is" button, or a mysterious "bring a dead body back to life" device?)

In any case there are things who physical properties we know, and which exist or have existed, but would be very valuable to create in laboratories. Like creating a string of DNA to order, which can then replicate itself into ordinary non-transmuted matter - very valuable in 1993!

Yeah, Harry discovered that you can't transmute something that hasn't already been created through more conventional means.