There's tons that no longer exist as independent entities. Of the Big 5 movie studios, only MGM is still independent. 20th Century Fox is under Disney, Paramount is under CBS, and Warner Bros is under AT&T, though they retain some identity still.

But RKO is just straight up dead, which I don't think anyone would have called in 1943, its 15th year. 1946 would be its most profitable year. It was gone in 1957.