He was 210 because that's what happened to him without hormones-he had an unusually intense reaction to them.
As far as what might've happened had he stayed around, it's just speculation. There are lots of similar what-ifs from then.
What if:
-Sergio hadn't been blacklisted and had competed in '73 and '74. No doubt it was the threat of him entering those shows that drove Schwarzenegger's conditioning those years.
-Robinson, Padilla, Szkalak and Mentzer hadn't been driven out of the IFBB by suspect politics.
There are a lot of ifs, keeping in mind also that the IFBB is a commercial organization with a lot of conflicting agendas that have little to do with the best guys winning.