Not going to happen. Doesn't matter if Mercedes offer them free engines, with Wehrlein and Stroll next year Williams would be utterly screwed and lose more money through the ensuing constructors championship drop than they'd saved on the engines, especially if Martini jump ship too.
They are going to drop anyway next year. They were only 5th this year and with multiple teams with issues.
Williams had it tough this year losing by a large gap in the end to FI and that is with Mclaren, TR, Renault and Haas all hamstrung by one thing or another, a lot of those teams problems will either improve or go away next year.
I think Williams are on for 7-8th next year regardless of the drivers they have. Stronger than Manor/Sauber and probably for another year or two Haas, but weaker than most other teams with similar budgets.
Haas had new team-itis, bad setup, incredibly inconsistent car and lots of stupid reliability issues with brakes and other things. A year of learning and moving to new parts/different brands of brake disc next year will improve them hugely but likely not enough. But they are now building a 2017 car with dramatically more knowledge than they had building the 2016 car.
TR will have a huge engine upgrade, Renault inherited a pretty bad, unreliable and not fast car, then had to stick a new engine in it and the whole package was bad. The renault engine itself improved hugely as evident from RBR's performance over the year. Renault should have been focusing on 2017 car rather than dumping big cash into 2016's dead end car. I see a huge jump but not necessarily big enough for next year.
Williams have been stagnant for 3 years, Symmonds is just a joke, he built the single most basic and non-complex car possible and had no clue how to genuinely improve it. Strategy guys and decisions made throughout races were terrible, tire testing and lack of a tire specialist till late this season, all terrible management. Even if they get Lowe (afaik only a guess/rumour right now) it won't be late enough to have a huge effect on the 2017 car nor the budget available to make huge changes in the team quickly.
They don't have a good aero design team, they did well enough with the no aero concept in a engine limited period of regulations where all the competitors screwed up their engines and other Merc engine teams had their own issues. Moving into an aero period of F1 where aero changes will be biggest.. I can't see that the current team and designers are going to come up with something good.
It's also why William's won't be an attractive seat to many, Button come back to be in a team probably behind Mclaren next year, doesn't sound likely does it, Massa to come back for another year... in a steadily less competitive car?
Williams is an upgrade for guys at the back of the grid or not driving, but not an attractive drive for people looking to come back to F1.