I feel like Mickey Mouse is really a special case - Disney employs literally thousands of people promoting their IP, and MM's been in constant ongoing and heavy use by them since the start. Mickey Mouse isn't a 'sitting on their laurels' situation - it's an actively worked bit of IP - not something that would be described as 'passive income'.
Wikipedia says Mickey Mouse was created by Walt Disney and Ub Iwerks. Walt Disney died in 1966, but Ub Iwerks only died in 1971, so wouldn't it still be a few more years before that 50 years after death runs out?