I do not understand this proposed change to the Terms of Service (ToS).
Stack Exchange cannot revoke Creative Commons licensing rights that have already been granted. Therefore, they cannot "un-license" any existing user profile web page content or other user-contributed content that has already been published under CC BY-SA.
However, that appears to be exactly the intention of this proposed ToS change. Hence my confusion.
In short, the intention seems to be to use the ToS to impose terms that directly contradict the license under which the data was published.
If I have misunderstood, please correct me.
If I haven't misunderstood, then the proposed new ToS amount to little more than an expression of vicarious licensor remorse, at least in the case of all existing profiles. By being inconsistent with the perpetual terms of the existing license, the new ToS would probably be unenforceable in this regard and therefore somewhat pointless.