It is practically impossible to make it through the day without seeing or hearing someone use “if not” to mean “maybe even,” as in: “Michael Jordan was the best basketball player of his era, *if not* of all time” when what’s clearly meant is this: “Michael Jordan was the best basketball player of his era, *maybe even* of all time.”
We will lose this usage battle, alas, and “if not” will join that list of words or phrases that are their own opposites.
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The authors of The Hobo Ethical Code (1889) used the phrase “if not” poignantly (and correctly):
When jungling* in town, respect handouts, do not wear them out, another hobo will be coming along who will need them as badly, if not worse than you.
The Code did not ask its hobo readers to imagine that others might be even worse off than themselves, for to do so might play down the troubles of someone who was at rock bottom, better off than no one.
Now read the quoted sentence again, replacing “if not” with “maybe even” – and experience how different it feels.
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*About the “hobo jungles.”
basil.CA 
Addendum: “My colleague noted, ‘Basil, you are a great, if not the greatest, blogger of all time.'”