After withdrawal is completed, your node would no longer be in the set of active validators and from that time could not validly propose a block or submit an attestation (or, more accurately, be selected as a block proposer, etc.)
I can still propose blocks invalidly, you see. And then someone who doesn't already have the consensus (e.g. trying to sync) will have no way to tell which is legitimate.
This is the problem - you can't look at what the system does when everything's working as it should, you have to look at what happens when it's outside of the comfort zone.
Even a relatively light reading of the Annotated Specs for Eth2[1] and/or the Eth Org's Proof of Stake FAQs[2] suggests the designers (and independent implementer-teams who gave feedback to designers, who gave direction to the implementers... lather, rinse, repeat) understand it's important to consider the overall system "outside of the comfort zone".
