He was a face of humanity for the Borg, nto a face of the Borg for humanity, and it wasn't something they did to stray away, it wasn't Patrick Stewart was coming back, so they needed a way out.
Can you explain what you mean, because I believe it to be the exact opposite. The Borg assimilated Picard specifically so they would have a unified face and voice to present to humanity. Picard was the face of the Borg for or to humanity. Picard spoke for the Borg and gave humanity someone to speak back to.
If he was the face of humanity to or for the Borg, he would have stayed human and not been assimilated. He would have been used by humanity and the Federation to speak for them to the Borg. At least, that is how I take your sentence.
Please clarify.