It is not clear what happens to user accounts after 2 years but we are assuming that the company would expect the user to upgrade to another HTC branded device to keep the service active.
Two years is also a very long time. Dropbox accounts might become cheaper or free for 25GB storage. Google is also expected to launch their Google Drive service in the near future which could become a solid alternative.
HTC expanded their collaboration with Dropbox to offer a platform similar to Apple iCloud on their Android range of devices.