The Metallica Problem occurs when an information-product like music, books, or cinema adapts to what its audience wants, which is by definition less focused than what the original creators envisioned, and so you end up with the same old slop that made shows like "C.H.I.P.S." and "Happy Days" so bonebrained moronic back in the day.
The fact is, the 1976 movie was fun: a triumph of integrating character drama with humor, action, and Buck Rogers style sci-fi. Everything since has been mediocre but because the audience was there with lots of money to hand to them, the studios cruised on it.
What happened in 1999 or so was that The Metallica Problem took over and Star Wars became standard slop with a sci-fi adventure flavor, even with Lucas at the helm. The fanbase shaped the movies into the usual junk without intending to.