In that case I agree, I'm sure there's plenty of ways to make a great film out of a purely invented MacGuffin, even crystal skulls.
But I'm not arguing about how great a film COULD be using an already debunked MacGuffin, I'm pointing out that we have the evidence in KOTCS - it's the only film in the Indy series that does this, and it's pretty average at best, and considered by many to be the worst of the 5.
Granted, there are many reasons why Skull isn't as good as the others, but a hodgepodge story around a weak and poorly thought out macguffin is one of them.