That's not something you would have to worry about because you, your children and their brand children would be dead by the time a normal person would be able to reach the point when the flash drive have reached it's limit read/write limit.
So basically that's not a draw back one would have you worry about. Your HDD drive on the other hand is very prone to failure, specially if it's in a portable device.
On normal use you are right a flash drive would take forever to die. However as the main storage device for a computer where its accessed all the time. Where the page file lives on it and virtual ram you will be writing to it hundreds of times a minute if not more. So actually your estimation is a bit off I think.