Well, releases are necessary to prevent a too high pressure buildup within the existing concrete containment vessel while cooling the core. A concrete sarcophagus would mean stopping the cooling down of the core, so it would have to be both watertight (or steamtight) and able to withstand high pressure and temperature. And it wouldn't be easy to build in a devastated area.

"People only accept change when they are faced with necessity, and only recognize necessity when a crisis is upon them." - Jean Monnet