Terry Murray of The Medical Post in Toronto offers:
Here’s the overworked cliche (mostly heard in TV news) I love to hate: something “changed his life forever” (and variations thereupon). “Forever” is a long time, and usually these statements are made well before the end of “forever.”
When I worked for the Globe and Mail’s Report on Business here in Toronto, back when dinosaurs roamed the earth and Grover Cleveland was president, we weren’t allowed to use “all-time,” as in “the stock his an all-time high,” for the same reason – “all time” wasn’t over yet.
I think you’re being a little tough. “Forever” is used as a synonym for “permanently.” If you have your legs amputated, it’s okay to say your life has been changed “forever.” Think of the alternative: “The amputation changed my life to date.”
Your strict rule would rule out expressions like, “I will be forever grateful to my parents for their goodness to me” on the grounds that one’s life doesn’t last “forever” (afterlife excluded).
The objection to “changed his life forever” is that it’s hackneyed, not that it’s illogical.