That's not how these contracts actually work. Certainly Amazon COULD act against their customer's interests but the customer will be around for a long time, so that would be really short-sighted.
The process for re-bidding them starts a long time before the contract ends, and often the incumbent will offer a temporary contract to cover the time between the end of the contract and the beginning of the next contract. The process is unpredictably long because one of the losers -- especially if it is the incumbent -- may protest the award.
A similar thing happened with a contract I worked a lot with and the new contract (much smaller than JEDI, but still ~$2BN). The contract was to replace legacy datacenters with cloud services and service management layered on top, and was an IDIQ (Indefinite Delivery, Indefinite Quantity) contract -- it was held up in protest so long after award that the government just ordered nothing, and came up with alternate strategies. The protesting entity was one of the winners of the bid (unlike JEDI, it was awarded to 4 parties, though they were all reselling AWS, Azure, Google, etc) and they ended up with nothing from this very expensive and time-consuming bidding process.
The process for re-bidding them starts a long time before the contract ends, and often the incumbent will offer a temporary contract to cover the time between the end of the contract and the beginning of the next contract. The process is unpredictably long because one of the losers -- especially if it is the incumbent -- may protest the award.
A similar thing happened with a contract I worked a lot with and the new contract (much smaller than JEDI, but still ~$2BN). The contract was to replace legacy datacenters with cloud services and service management layered on top, and was an IDIQ (Indefinite Delivery, Indefinite Quantity) contract -- it was held up in protest so long after award that the government just ordered nothing, and came up with alternate strategies. The protesting entity was one of the winners of the bid (unlike JEDI, it was awarded to 4 parties, though they were all reselling AWS, Azure, Google, etc) and they ended up with nothing from this very expensive and time-consuming bidding process.