considering a lot of factors, I think that authors have the right to make these decisions. MIA is not an excuse to infringe upon copyrights (which last an awful long time AFTER a person is declared deceased)
Usually the author has discussed it with someone, or left instructions, wishes, or other communication of their wishes. In the event of a sudden death, I feel it is up to the holder of those works to decide what to do with them whether it be the family or a publishing house. Also, depending on the circumstances of the works... some things just do not need completion. Sure there will be some readers who bitch about it (whiners all!) but if it's not finished, well it's not. However, there are also some things that do need to be finished (the exmaple of the Wheel of Time for example). The work will not be the same, but it can be similar, and sometimes that will just have to suffice.