This is a good question to ask. If an app is in the Windows Store and the developer's account with the Store expires after some time (like in a year), will the app disappear or what?
At present, apps will no longer be visible once a developer account expires. However, customers who already own the app will still be able to reacquire it later on, as when setting up a new device, but others cannot see the app. In-app purchases are also disabled for obvious reasons: the developer account is how revenue transfers from purchases to the developer, so if that account has expired there's no longer a legitimate channel for the revenue stream.
You will get an email notification if your account is near expiry, so be sure to keep a current email listed there.
Also note that if your account expires, all reserved app names will be released.