I don't believe it is correct to describe the 9F as either experimental or not successful. In fact they were remarkably successful locomotives. As George says, the only reason they were scrapped early was because of the unexpected acceleration of BR's dieselisation plan. The last 9F (named "Evening Star") was built in 1960 and taken out of service in 1965 (fortunately it was preserved as part of the national collection and not scrapped), a service life of only five years for a loco that had been designed with a working life of forty years or more - under the original modernisation plan, steam would be gradually phased out over that forty year period and replaced with electrification, skipping dieselisation completely.
I think you have picked up on a Wikipedia article which refers only to ten of the 9Fs which were built with Franco-Crosti boilers, "forming a subclass" (Wikipedia quote). Indeed this subclass was experimental and was not deemed to be successful, but the other 241 locomotives of the main 9F class were remarkably successful - to quote
the main Wikipedia article on the 9F, "It was one of the most powerful steam locomotive types ever constructed in Britain, and successfully performed its intended duties". Indeed it also performed well on other duties which were not its "intended duties" - designed as a heavy fast freight loco, it also found employment on passenger duties.