CrazyMom also noted that the life source was finite. She had each person has some in them, but others want more. And this light can go out.
After MIB was drifted into the light cave, the light did dim. And if one believes the cave is where Adam and Eve was found was the same, then the light did go out.
It does put into play the notion that the universe is not infinite but finite. Life as its own element is not replaceable. It can be depleted. Like a light, be extinguished. And maybe that is the warning that people will "cease to exist" if the EM energy, life source or the island is compromised by Smokey.
A gnawing question arises to why, if there is such grave danger of allowing the life force to escape or be compromised, did Jacob allow visitors to play with it?
Claudia's people were smart enough, according to MIB, to start digging for the substance, and theorized that using a wheel and water, they could use the energy to return to their home. How they figured that out is a mystery. These visitors were killed off before attempting the Turn.
Unless Jacob was really hands off, in total isolation, the other people brought to the island continued the work of Claudia's people. They continued to dig wells, and in one deep one (under the Orchid station) completed the FDW room. Based on the construction, it looked ancient, pre-Industrial revolution. That puts the inventors in the realm of the Greeks, Romans or Egyptians, whose advanced, ancient mechanical knowledge is just being understood by modern scientists.
So, some distant past group completed the FDW, and turned it. It had to work because Ben knew about it, how to operate it, and what the consequences of the turn was to the island and to the turner. (He originally said that turning the FDW would move the island and the turner would never be allowed to come back. We saw evidence of the island disappearing, and Hawking's station trying to re-locate it. However, Ben did return to the island.)
So the physics of the light in conjunction with the FDW were known by previous inhabitants. So why did not Smokey leave with the first or second turner? Apparently he cannot leave the island by that means. Maybe, because Smokey appears to be created from the soul of man and the life source radiated waters, it cancels itself out. MIB's memories, soul, essence is trapped in the smoke monster form, which is "worse than death."
But what is really strange is that Jacob and/or MIB allowed more people to come to the island and tinker with the EM light source. The Egyptians probably used a pool for some healing religion properties. The military may have come in order to compare it to hydrogen bombs. Dharma may have come to test its properties on animals, humans, and as a power source.
If you are supposed to protect the island (and the finite life source contained thereon), there is little reason to bring more and more destructive people to the island to mess with its equilibrium.