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I like this explanation best.  Temp tracks are apparently not the reason for Marvel's music.
 
It's all Hans Zimmer's fault.
 
https://www.youtube.com/watch?v=UcXsH88XlKM

A very good video!
 
I still feel though, that both these videos are missing the real reason (even though it was briefly touched by this one): money. Sure using computers and DAWs has changed music making for ever, but film music is not the only area affected by this. The big problem with Hollywood that people seem to forget is that they aren't in it to make art, they in it to make money. And that drives their choices (of course there are exceptions, there always are).
 
Take Star Wars for an example; can you honestly say that it is the greatest piece of art ever created (the film itself)? I would not say that. Is one of the most successful pop culture moneymakers? Yeah.
 
I'd love to see Hollywood venture more into the unknown, because when they do, this happens: https://www.youtube.com/watch?v=oCvLUxICxEI
 
Yeah, Zimmer changed film music. But so did Ennio Morricone. And Bernard Herrmann. And Max Steiner. Do people like the change? Some like, some don't, a lot people really don't care. But blaming things on computers and one composer, I find that ridiculous. It's the people making the final decision that produce the final product. Zimmer himself has commented that he does not want to produce the sound he's famous for, but that is what studios want from every single composer nowadays, because that sound made film music accessible to the masses. And saying that Zimmer is just about digital is stupid. Sure he writes the stuff on computers, but in the end he employs skilled musicians for the final product.
 
What is sad is that they ask other people to imitate, because it's the thing that makes them money.
 
"Boys, you must strive to find your own voice. Because the longer you wait to begin, the less likely you are to find it at all." - Dead Poet Society, 1989.