Simpsons plagiarism?

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"The Simpsons", "Family Guy" and "American Dad" are very similar TV programs. Shouldn't it be enough of that? I mean, The Simpsons have succeded through many years, Family Guy is another great show based on a similar idea, but with a change, which was the talking dog, who added a non-human influence in the program to make it more interesting. With American Dad it is all the same as Family Guy, it didn't include nothing new (non-human character is replaced by an alien instead than a dog). What do you think about it?