I don't think anyone thinks Cesar will be here past free agency. But, apparently, 2B is not a position of scarcity around the league and therefore his value in hand has consistently exceeded his trade value. I would also go so far as to say Kingery's value (be it in hand or trade, not that he's getting traded) is greater for playing multiple positions. Guys like Zobrist (before this year), Merrifield and Taylor are more valued than guys who just play 2B or 3B (but for the cream of the crop).
It will work itself out soon enough, whether in a month or six months. Kingery could be our 3B or CF next year, he could stay in the utility role that includes those positions, or they could hand 2B to him.
Seems to me the main problem with the super-utility guy is 3B tends to be their weakest position (or they can play it but not well). They all play middle IF and CF and can manage at either corner OF spot, but the arm and the footwork doesn't translate the same way. Miller doesn't really play it. Merrifield rarely plays it. Dietrich not good defensively.
It still would not surprise me if Kingery is never truly the starting 2B, even as Cesar moves on. Who knows what the roster will really look like (or even who the manager will be) in 2021 but right now it wouldn't surprise me if they acquire or develop more Swiss Army knife type guys.