What may happen if oceanic plates stop moving

There would be no new island formations when oceanic plates stop moving.

Explanation:

The oceanic plate or the tectonic plate movement determines the several activity of land and ocean which keep making the life possible. As the plate movement stop, volcanic eruption will be stopped, sea floor will not spread further, every activity below ground will cease.

The pressure increases inside the core and so does the erosion. The continent will be lost with all its life on the land. Oceanic life will survive more but may freeze the planet to die.