On a long enough time scale, sure Amazon will fail. But it definitely won't happen within five years. Or ten. Or probably even thirty. Amazon's not too big to fail, but they are absolutely too big to fail soon.
One of their secret weapons is Amazon Web Services:
AWS is huge. It powers many of the world's largest websites. It powers significant portions of the US government. It even powers Cake. AWS has competitors like Google Cloud and Microsoft Azure, but they're nowhere near as big and don't offer nearly as many services.
Even if Amazon's retail business failed tomorrow, Amazon could still survive more or less indefinitely, and even continue to grow, based on their AWS business alone. And of course their retail business is in no danger of failing; in fact, they've dominated so many retail niches that this seems astronomically unlikely to happen anytime soon.
Amazon has too much of a head start and is still super nimble. Unless they either suffer from years of mismanagement or have the rug pulled out from under them by some huge business or technological shift (like how the appearance of the Internet disrupted the newspaper industry), I think their dominance is safe for many decades to come.