Many words are based on other words or combinations of other words, doesn't mean they're not words now. "reinstall" is certainly a word, doesn't matter how it came to be one.
Discussion
yes but the spelling is not important as the sound, it's two words when spoken, with a distinct pause between the prefix and the word it modifies, like antifascist - you say it anti-fascist, and at some point it got so common people drop the hyphen in the text
Like watermelon is also not a word but a combination of "water" and "melon"?
