I kind of agree but I really think that Senna could excel in a decent seat, especially as you state with the williams getting the new engine. I think Rubens has had his day, he made some pretty silly mistakes last season which someone with his vast experience should have known better. It depends what Williams are looking for at the end of the day, development or outright points. I agree that Rubens with his knowledge can help to develop and your idea of a 1 year drive is a fair point. I do also believe however that Senna could blossom in a decent team, with decent development and eagerness for past glory.
It will be interesting to see how things progress