Trademarks protect the consumer as much as the buyer. The only reason someone would want to put two "golden arches" on their burger joint is to make people think it's mcdonalds. That's bad for mcdonalds the company sure, but it's also bad for the customer who thinks they're getting a certain "standard", and do not deserve a counterfeit good.
The consumer is protected, the company has recognisable authority and the concept exists within public space.
I guess the biggest distinction between Sherlock Holmes and Mickey Mouse (aside the estates that own them) is that Mickey is a design as much as a character. The 3 circled motif is iconic enough that there is an ongoing spat between Disney and Deadmau5 -- though I do side with Deadmau5 on that, if just as a middle finger to Disney :D
I'm not suggesting law should stipulate that fictional characters need a design before they can be trademarked nor that Mickey Mouse shouldn't be trademarkable. Just that there is definitely an unclear grey area and I don't really know the answer to your question. But it is a very good question that gave me pause for thought.