That's what I thought too. At least Williams now has the back up plan in place to support Valterri's move. Get their engine discount and still be able to have an experienced driver for next season.
Anyway, if Mercedes is looking for a clear #2 to run behind Hamilton, then really should just give their own Jr drivers a chance. What would be the point of going thru all that trouble for Bottas?
If they're looking to score maximum amount of points with a proven champ next season and if Alonso is really not willing to sign on, then Jenson is really the best and simplest choice.