I think that's exactly what Mercedes are doing - putting Russell to test to see how good he really is for the future; they already know what to expect from Ocon, and to be fair to him, he's a good driver and would do well for Mercedes, but he could be a good replacement for Bottas, not for Hamilton once he retires...
I like this new trend of talented youngsters being given seats in F1 lately though. Verstappen, Gasly, Leclerc; now Giovinazzi, Norris, Russell. Good stuff.