Also, as previously mentioned, timing played a big factor in his success. I think if Arnold were to just be starting out now he wouldn't be the standout actor or novelty that he was back then. Arnold was the "perfect storm" so to speak to become the biggest action hero and most popular bodybuilder of all-time.
He had the work ethic, attitude, determination, physical stature and looks, accent, background, uniqueness, charisma, intelligence, and novelty factor among other attributes that contributed to his success.
Couple his attributes with the era in which he came up and you can see why he rose to the stardom he did. Although bodybuilding isn't considered mainstream today, it was even much more of a novelty or unknown sport or activity back then. Arnold and his fellow bodybuilders stood out even more back then because quite simply there were a lot fewer muscular bodies seen by the mainstream public when compared to today with the juiced wrestlers, athletes, actors, and a society which puts a greater emphasis on physical appearance including non-competitive bodybuilders and gym-goers.
Physique-wise Arnold would not be the novelty or as outstanding today as he was back then so he would lose part of his "wow factor".
I'm not saying Arnold wouldn't be successful but he probably wouldn't be able to reach the same level of stardom with today's society which has "seen it all" already. I'm just speaking of his career outside of bodybuilding. Physique-wise it's hard to say how successful he would be in bodybuilding today with a lot more competitors, and judging standards.
It is a testament to his greatness though that we are still discussing him now many years since he competed and he is still considered by many to be the greatest bodybuilder and action movie actor of all time!