There's three reasons that this won't happen:
1) Thomas got old in a hurry and doesn't play as well as he did before.
2) When the Niners were thinking of signing him, Thomas said he wanted to stay on the East Coast to be closer to his family. Last I checked, the Niners are still on the West Coast
3) Briggs is younger, better than Thomas right now, and can play special teams.
End of story.....