People have been free to come and go from the Island without causing such drastic effects. Why do the O6 have to return? Some points, first:
- The Island has special electromagnetic properties that bend space and time around a central point and includes all matter within a radius around that point. This point is in The Orchid, where the frozen wheel lies.
- Moving the wheel changes the level or intensity of the electromagnetic properties of the Island, and thus the radius of the space-time bend.
So the theory goes: When Ben started moving the Frozen Wheel, the O6 were on the perimeter of the Island's electromagnetic radius. But when Ben finished moving the wheel, the O6 were outside of the radius because they were moving along at a rapid pace in a helicopter. The O6 were initially in, then out, of the radius while it was changing due to Ben's moving of the wheel. They are supposed to be on the Island because they were a part of its space-time continuum but jumped out of it, causing a disruption of space and time on the Island. The Island has literally "Lost" the Oceanic 6. The universe and Island are now trying to "course correct" and find the Oceanic 6. The flashes Sawyer, Locke, Juliet, and the others are experiencing on the Island are attempts to course correct and find the O6.
Until the O6 return, the Island will continue to search for them. It will expand its radius of space-time with each flash, causing the flashes to get closer together and more intense. As we saw with Charlotte Lewis, this will kill people. Eventually, the flashes will get large enough to encompass more and more people, and bring about the destruction of space and time on Earth as a whole, killing everyone.