Each year there is a new version of Elements released. You don't have to buy the new version if you don't need any bug fixes or additional features added to the new version and want to continue using the old version.
If you want the new version of Elements (which is a separate program and can coexist on the same computer as the old version), you will have to buy it.
The Creative Cloud Photography Plan is a subscription and you may download the newest versions (and also keep the old versions if you wish, as long as you continue the subscription to the plan) when they are released.