Amazon's Prime Day has a problem: Almost everyone has Prime already

Amazon has nearly maxed out the addressable market for its Prime membership, an analyst said.Bloomberg/Getty ImagesAmazon launched Prime Day to get more shoppers to sign up for its Prime membership.Now, almost everyone who wants to be a Prime member already is.That's changing the kinds of deals you can expect to see during this year's Prime Day.Amazon Prime Day has served its purpose well — perhaps a little too well.Amazon launched Prime Day to get more people to pay for its memb