Of course she has to come back! She's pregnant with Matthew's baby. We still don't know if Claudia will give birth or miscarry if she ever returns. On the other hand, I think making Claudia crazy was pretty much the only way to write her out temporarily until she could come back on a later date. And Sammy Jo actually has a solid character arc: he's dealing with severe abandonment issues by his father and he's trying to change and be a better person for Steven. So yeah I do believe he has a purpose because Sammy Jo & Steven actually have great chemistry together. Besides, who else are they going to put Steven with? Ted's already gone, so for the time being he's stuck with Sammy Jo.
I think for a potential season 2 there's a good chance that maybe Cristal might be recast with a better, stronger actress that can keep up with Nicollette Sheridan. And maybe Steven will also be recast too. If I were keeping Jeff around I'd recast him too. But honestly, the Colbys need to be gone. They've run their course already.