How much content that you put online ten years ago is still accessible?
Even if Quora lasts forty years (!), why would they bother keeping your content accessible? People complain about being shown three year old content in their feeds now. There won’t be terribly much interest in content that old, which would only be of historic interest. It would get in the way of discovering new content, and there is a cost behind keeping content available on the live site. Quora may or may not keep the content in archives, but I think it unlikely you could pop back in to Quora and read 40 year old answers in your profile.
Even if Quora lasts forty years (!)
We are in an era now of obsolescence, of erasure of our histories. I pity anyone trying to do philology on contemporary texts. Archive your content: your hard drive is the best guarantee of permanence that you have—because you yourself will be motivated to upgrade your computer every few years, and copy your stuff across.