Not having CLA or equivalent prevents this kind of thing. DCO is for projects that want minimal legal process without copyright assignment. For the record akka has CLA.
CLA is sort of neutral in this regard. You can have DCO-style CLAs with incoming == outgoing licensing, where the contributor and the project organization end up with the same rights. But like with DCO, switching to proprietary is still possible in principle if the license is sufficiently permissive. And admittedly, such symmetric CLAs are somewhat rare; DCO has mostly taken over this space.