First, let me make a grammatical observation: You should be asking for PRINCIPAL Product. (The word principle pertains to a basic tenet such as "The Principle of One Man, One Vote." OOPS--I should say "One Person, One Vote--if of age 18 or more.
If I were to answer the question, I'd opine that the principal product of the modern film industry is the SEQUEL to a blockbuster violent fantasy film. Which is why I rarely attend movies anymore.
In the golden era of movies (the studio era), I would say that the principal products then were the narrative--so many great films had great screenplays, often based on the greatest novels of literature. This literate, artistic temperament was often attributed to those great studio heads, several of whom fled Hitler in 1930s.
I'd welcome feedback from this. Thanks.