I think the die was cast when he went on loan and it failed so spectacularly... AW didn't put enough trust in him or clearly make him feel he had a legit role here as he was starting to get impatient and he decided to leave. It isn't black/white imho.. had AW handled it differently from far earlier, things might have turned out different.
In this particular case, I think it was perhaps a miscalculation on AW's part, but it is perfectly reasonable to acknowledge that is hindsight and give him a pass on this one.
Serge is gone - he wanted to leave in the end and good or bad, that is the situation we have... would I rather see him than Ox? Yeah, probably, but that has more to do with Ox than it really does Serge at this point (even though I think Serge is the more natural footballer and likely to have better career).