During Star Wars the Clone Wars, there is an episode where the Senate is debating deregulating the banks and having to finance more Clones. IIRC, Padme protests against this by talking about how the cost of the Clone Army has cause structural readjustment on Coruscant, with some levels losing power and other basic necessities.
I think this also shows why the Clone needed to be nixed. The Empire needed to create a force that could both be omnipresent in Imperial worlds to create order through fear AND reconquer the remaining separatist holdouts. This is also while in the midst of overhauling the aesthetics of the Empire, including new uniforms, starships, vehicles, etc. I think with all of that, the Empire would simply buckle under the cost.
Further, the Empire relied on increasing centralisation around the Core Worlds. Deregulating the banks and relying on the Kaminoans for their army would leave them vulnerable to overthrow. By getting rid of the clones, the Empire can concentrate social mobility into the moving up the military and navy into higher parts of the state apparatus. It provides jobs, security, and education to many people, especially the disadvantaged worlds in the periphery. This further secures Palpatine’s legitimacy.