The number of articles which Wikipedia can have is limited by the number of at least marginally notable and verifiable things which exist. After every notable person, place, movie, scientific topic, and so on have their own article, we'll run out of things to write about. Whether this will happen at 2 million, 3 million, 4 million, it will happen.
On the other hand, the potential number of lists is almost infinite, as there are a virtually unlimited number of ways of slicing up portions of reality.
For example, one could make a "List of birds native to the Chicago area". If it were nicely wikified and contained a reasonable number of items on it, it would certainly not be deleted.
However, what you've actually done is create the first example of "List of X's native to the Y area", where X is any one of dozens or hundreds of types of creatures... insects, mammals, amphibians, trees, flowers, and so on, while Y is any one of tens of thousands of towns, cities, states, counties, countries, provinces, geographic areas, and so on.
Multiply 100 X's by 50,000 Y's, and you get 5 million lists.
Take the roughly 500,000 of our article topics which have been mentioned enough times in popular culture to make up a list, and you get 500,000 "X in popular culture" articles.
Take every one of the tens of thousands of diseases which exist, find evidence of notable people who have had the disease, and you get 25,000 "List of people who have had disease X" articles.
And so on, and so on, and so on.
Even removing the obvious silly and ridiculous lists, like "List of blind left-handed Canadians who have worn blue jeans" still leaves us with more reasonable sounding lists than any of ever imagined we would have.
End result: en.Wikipedia, in the year 2050, will have 5 million articles, and 500 million lists.
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