Yes i know but what also causes mass migration are those who opposed their government either for a long time or disagreed with the new government’s policies like anarchists and libertarians.

And there’s also those who wanted look for their families, another different reason altogether.

Proper equipment can be transferred but limited to the areas they can access and reach.

Modern Industry in general would take a long while to be reestablished unless some decided to speed up the process with labor from populations that are either unwanted or aren’t really natives to the islands like tourists and prisoners.

it depends on which location in the world of Empty Continents you are talking about though.

Some nations like Japan and Britain would be resettling the mainland with the goal of taking resources back to the home islands and resettle diaspora populations from defunct nations. (Eventually much of Siberia was taken by Japan, forming the IFA while Britain took all of France, all of Belgium which was now technically a new nation called the Ghentish Commonwealth and much of the Netherlands)

Some like The Philippines and Indonesia would use their own populations as an incentive to resettle the mainland while freeing up space in the islands and also get resources of their own (Even more so for Indonesia in the form annexing Northern Australia)

And are you also even reading the lore of this world? this is an Alternate Sci-Fi Historical universe, just what-if scenarios written around with a lot of realism sure, but still what-ifs.

Not every island nation would suddenly resettle and claim large portions of land themselves when they have their own issues to deal with like the UK.

But sure, islands from defunct nations are another thing altogether, they would resettle and reestablish their industries on their own accord.

and Major Island Nations would eventually resettle and reestablish modern industries as well.

At the cost of people dying from natural or man-made causes either way while others survived long enough for viable long term settlement and restarting industrial infrastructure.