In A new hope Luke mentions that one of his friends, Biggs Darklighter was leaving for the Imperial academy and that he wanted to go. Of course Bigg then showed up in the Rebellion.
The Clone army for all the Gee Whiz factor had three fundamental flaws.
1st without genetic diversity any population is doomed to illness and death. a few minutes in a suitably scifi rebel genetics lab cooking up a smart virus and presto Clone genocide.
2nd The Clones came from a singular Source, If the Clone growers turned on you, You loose new troops. For Palpatine's needs at the start of the Empire he needed a Army from which to build his empire.
3rd What do you do with the old clones? Millions of the same man over and over... Do you give them rights and Retirement? do you kill them off? Basically you hit a point of decay much like what happened to sparta.
To expand beyond the first few generations the Emperor needed fresh troops and a diverse population growth. So he may have kept some of the old Clones around as trainers and more and more senior officers but eventually he used their tech to found a more traditional Army and Navy. Then as The Emperor fell and his Successor and the First Order rose, They adopted the Janissary model with strict indoctrination at birth.
