I think that would've been the better choice, even if it meant that people new to the series don't get to see how the orcs came to Azeroth. One of my fears about this movie when its plot was first announced was the pacing issue, that they had to switch a lot around the Alliance and Horde. And this became a major issue in the movie after all, which resulted in some very fast pacing with only very little time between scenes and very little buildup to anything. As a result, every death in this movie felt emotionless and they all just felt like throwaway characters. Now I see Legendary trying to hype up the ''Alliance vs Horde'' thing as some kind of promotion, which I guess is a good marketing stunt that wouldn't have worked as much in an Arthas movie.
But if they did make an Arthas movie instead, we'd have a more linear storyline which would have a lot more breating time to flesh out characters and the universe. People woud have a better chance to get into Arthas as a character, with everything he's going trough. And secondary characters like Jaina, Uther and Muradin would've been great additions to the cast as his love interest and 2 mentors. It may seem more like a cliché fantasy, but it would really work.
Its like i said before, a movie franchise does not necessarily have to start at the very beginning. I name Star Wars as a great example, that basically started out where the good stuff happened that made the franchise popular in the first place. And it was like 15 years later that they started to make a prequel trilogy, so that everyone had the opportunity to see how Darth Vader became the character they knew, and how the Jedi had fallen in the first place.
I'd call BvS a better movie than Warcraft, despite both of their flaws. I see a common complaint about BvS being actually about their pacing issues, but I'd say its far worse with Warcraft.