Well David Chase never intended for the series to surpass five seasons let alone six and possibly a film. If Chase wanted the actors to reprise their roles it would only take a lot of one thing...MONEY! Actors who say they have lost desire in a show is their way of saying, "I want more money!"

"When my time comes, tell me, will I stand up?"
Paulie "Walnuts" Gaultiere - The Sopranos