Star Wars: The Bad Batch episode 6 suggests the real reason the Empire abandoned clones. Set shortly after the events of Star Wars: Episode III - Revenge of the Sith, Lucasfilm's latest animated series appears to be answering one of the greatest mysteries of the franchise; why did the Empire stop using clones? Fans would've expected the Kaminoans' clones to be perfectly suited to the Empire, particularly given the proven effectiveness of the behavioral control chips. For all that's the case, though, the Empire seems to have wasted little time decommissioning the clones.
Star Wars: The Bad Batch has seen Moff Tarkin head to Kamino to decide on the future of the clone program. Tarkin was never overly fond of the clones, even during the Clone Wars, but his stated rationale was particularly amusing; he claims to believe the clones are too expensive. He serves in an Empire to whom money is no object, with Palpatine already working on multiple superweapon projects, so suffice to say Tarkin's argument doesn't fit with Imperial philosophy. While the cost may be a factor, the reality is likely to be a lot more complex.
Star Wars: The Bad Batch episode 6 offers an important clue to why the Empire shut down the clones, depending instead upon a legion of stormtroopers. In this episode, Clone Force 99 encounter sisters Rafa and Trace Martez, who have apparently become part of a nascent cell of rebels - although it's unclear who Rafa and Trace are working for. Significantly, Rafa and Trace leave Corellia with the data from an old Separatist tactical droid, intelligence designed to help fight the clone army. This is information the Empire wanted destroyed, and it is circulating among the Empire's enemies within just months of Palpatine's taking the Imperial throne.
The Separatist tactical droids were strategic geniuses, possessed of advanced analytical abilities and capable of adapting to the strategies used against the droid army; in fact, some had even learned how to counter specific Jedi generals, able to identify who they were dealing with simply by observing their tactics. By the end of the Clone Wars, the tactical droids had a deep understanding of every single strategy deployed by the Grand Army of the Republic, and without Palpatine's manipulation of events it is entirely possible they would have triumphed. Rafa and Trace's successful acquisition of the data from a tactical droid undermined the entire clone project, because it meant the Empire's new enemies - the nascent rebel cells, such as those established by Saw Gerrera - already knew how to beat the clones.
This, then, is likely the ultimate reason why the Empire decommissioned the clones; because they ceased to be of use. In theory, the stormtrooper program would allow the Empire instead to choose a new wave of tacticians who could encourage a different set of tactics and strategies, making them much more effective against the rebels. Of course, in reality the Empire introduced new dependencies, because it required skilled tacticians to come up with these new approaches. As seen in short stories published in Star Wars Insider, such strategists were few and far between, simply because galactic conflict had been unheard of before the Clone Wars - and the Jedi were the ones leading the clones during that war. In the end, the average stormtrooper proved far less skilled than your typical clone trooper, fortunately for the rebels.
