The PT may have influenced the marketing trend of First Of An Epic Trilogy... It also may just be the fact that the trend was being born around the same time.

That said; It seems far worse now, ten years removed. If I really had to pinpoint the genesis of the Epic Trilogy In The Making, I'd go with Lord Of The Rings. If the PT had any impact, it was that it showed story and acting aren't terribly important.

As Boost pointed out, these days the concept is pushed onto the audience before the first film is even released. It's as though the studios have decided all it takes to get people to buy into the franchise is to tell them it's Epic before they even see it. It must work to an extent because they keep doing it.

If you look at the film The People vs George Lucas, several of the people interviewed specifically state they knew they had to see all three before they could write it off officially. By then the studio already had their money. The studio prolongs the mediocre product just long enough to ensure they have the money all but guaranteed.

I didn't do that with the PT. I saw the first one, quickly realized it was shit, and moved on. However, that's not typical behavior so it's a safe roll of the dice for the studios. Keep telling them it's Epic and they'll believe you. In fact, it's stated as fact in the trailers.

It's been explained before;

http://www.andersen.sdu.dk/vaerk/hersholt/TheEmperorsNewClothes_e.html