Clarifying Practicality and Usage of a DFA's Accepted Language
Picture the DFA $M$ as a directed graph, as the usual graphical depiction. Then the final states of your $M'$ that accepts prefixes of the language accepted by $M$ will have all states from which a final state of $M$ can be reached as final states. It should be clear that not all states of $M$ qualify. In particular, any dead states of $M$ (non-final states that loop back on all symbols) won't be final in $M'$.