I know it isn't a popular opinion but I don't think Kubica should get any F1 seat. There is just simply too much young talent to warrant a 33 y/o driver getting a seat.
The only reason I could see to put Kubica in the car would be for PR reasons, and aside from it not being what F1 needs, Williams really cannot afford it. In the same way that McLaren couldn't afford to put Alonso and his public criticism in the car.
And with Williams using the Mercedes power units, they might well be better off improving their relationship with Mercedes and with it not only improving their car and competitiveness but also get access to their drivers. The Mercedes Young Driver programme has Ocon, Wehrlein and Russell all of whom deserve a seat in F1 imo.