The cast can make or break the movie, even if the plot sucks. Transformers is a good example. Even with the special effects, the plot was pretty straightforward and cliche. Shia LeBeouf and Megan Fox made the movie shine.

I don't see why you'd think the plot will suck that bad. It would be like any of the DB movies, kind of random and loosely based on the main plotline of the anime/manga, but occurring just outside of that plotline.

To me, this could be a great movie with special effects and seeing DB in live action. It could be as cool as Transformers or the first 2 Spider-man movies. But if the cast is all wrong and they change too many details (due to the cast), it will fail the DB universe, even if it makes money for Hollywood.