The lack of stars was the reason Dune '84 flopped
Back then and even today it's really hard to succeed without big stars.I think De Laurentis thought that if Star Wars can pull it off with (relatively)unknowns (minus Alec Guiness) he could do it too.
Anyway it was a huge movie,maybe he couldn't afford an all star cast ,but if I could cast this movie back then, I would cast some stars in key roles to give it a boost at the Box Office.
Sean Connery as Duke Leto
James Earl Jones as Mentat Thufir Hawat
Marlon Brando as the Baron Vladimir Harkonnen
Charlton Heston as Emperor Shaddam IV
Burt Lancaster as Gurney(he is 70+ in the book)
Charles Bronson as Stilgar
And MAYBE a young Tom Cruise as Paul (even though I love MacLachlan's portrayal)