I think whether we keep Kolo depends on what punishment Sakho gets. And that's why we haven't made the decision yet.
There's two 1st-team-ready CB's guaranteed to be here next season - Lovren & Matip. Gomez, who looked the real deal before injury (but at full back) is back-up.
If Sakho gets 6 months, then I think we'll take it on the chin & retain Kolo as cover early season (Lovren, Matip, Kolo, Gomez, with Sakho to come back around Dec/Jan after getting match fit, and Kolo to take more of a back seat in the second half of the season.
If the ban is 12+ months, then I think we'll look to sign another CB, and unfortunately not offer Kolo a new deal (Lovren, Matip, new CB, Gomez).