Then you lose it. If after 2 years you decide not to pay for the added space, then they reduce you down to the free allotted space. You will have to remove/transfer your files to another source or risk losing them if you don't upgrade.
Drop box is providing an added 23G for free for two years. After that time you would either have to pay for that space or you would loose the space. As noted above you would have to move your stored data to a different location. Of course who know what it will be in two years things change.