It's definitely being rumored that they got him as a short term hedge with Klay out and then will trade him. one name already out there is Robert Covington, Minnesota wanted Russell before the Warriors trade, they have Covington. He's a 3 and D guy so Klay would move back to the 2 when healthy(hopefully) and Covington would be the 3.
From what the articles say the idea is the only way they were going to be able to sign a guy at that level in the near future was by doing this, sign and trade for Durant get a new guy to max and then either keep him or trade him down the line.
Also Draymond is again saying he's open to extending his deal now so their intention is still very much to keep the big 3 together. I'm sure if Russell plays well enough they might decide to keep him but I just don't see enough defense with him in there.