He himself once said that he was writing "fewer notes" than before. His scores grew more predictable during that phase. As me and others have pointed out before, there were many very good scores as well. But even those were more streamlined.
I remember seeing a piece on Goldsmith on a US morning show at the time of The River Wild. He was quoted as saying that he was "simplifying" his music. Now whether that means going genuinely minimalist or whether he just didn't want to continue writing so many notes in his old age who knows, but certainly his scores from then on did tend to lack some of the intricacy and melodic counterpoint of 60s and 70s efforts. Personally I think he was just getting tired, and that's entirely understandable.
The Mummy is not a score that sounds like it was written by someone "getting tired". And his final score, Looney Tunes: Back in Action, CERTAINLY doesn't sound like it. And he wrote that score while he was going through cancer treatment! But it's got all the zany energy of John Powell in his prime.
One other remark I do remember is about him cutting back on the tympany in his scores. I think maybe he felt he was in a rut, or maybe he felt other composers were aping his style (nah, couldn't be!). For my part I've admired Goldsmith from all periods depending on the individual films. I can listen to his stuff repeatedly, whereas Williams...it's good but once or twice is enough. Just my taste, of course.
If Mr. G were just beginning with the output of his "tired" years, hell, the chorus of hosannas about this new & exciting up & coming composer would be resounding all over the messageboard.
PS
"A long time ago in a galaxy far, far away...."
HOWARD: Want to know my take on your film score fast-food assembly line of late?
JERRY: No, but I bet you’re going to tell me anyway.
HOWARD (pulling back): Sorry. I’m really not like this. And I couldn’t care less if you did a hundred movies a year. Most of them stink and I don’t see ‘em anyway. (He hears Jerry about to start) --The movies, not you! It’s just that you remind me of those guys who rack up all the overtime and back sick-pay the year they’re retiring so they can live off the big pension when it’s over. (pauses, then softens his tone--) Or one day it hits ‘em that they’ve gotten old and they’re not going to be around forever. (pauses again, then--) Look. You’re 70. How much more time you got?
I remember the Oscar orchestra trying to hammer out a version of the Total Recall theme when the movie won sound design or something--people must have been thinking "What is THAT shit?" The orchestra just didn't have the layout to do that justice, and in any case it is a relatively simple piece of material apart from its orchestration (and of course the fact that it contains everything Goldsmith needs to build the rest of a mammoth score).
You're really not kidding about that Oscar arrangement: